The 55 - 250 can produce decent results, a reasonable lens for the money. The STM version is better. The EF 70 - 300 IS is better still, and of course L glass is better than most other things.

Main thing is to use it. If you don't perceive problems then it is fine. Probably not worth upgrading unless you have a problem and know what that problem is, or unless you have silly money to play with.

I would continue down your path of functional expansion, and consider used equipment. For flash a used 580 EXII could be a good option, 580 ain't cheap, but it is a very powerful workhorse. For macro there are many options, within the Canon range the EF-S 60mm macro lens is excellent, but many prefer the greater lens to subject distance of the EF 100mm macro.

I started with pretty much the lenses you have, which my wife now has on a used T2i and is very happy. Expanded outwards into many things, some quite esoteric, but the three that I rely on most now are the EF-S 10-22 zoom, EF-S 18-135 STM, and EF 70-300 IS. Plus the 60mm macro, 50mm f/1.8, a Samyang 8mm fisheye, and some modified old manual focus Canon lenses, but that is a very different story.

It all costs money, and only you can choose what you will get value from.

Here's an early shot taken with a T3i and the EF-S 55-250mm IS II, opportunistic and hand held, pretty much SOOC....

If that works for you, then the lens and camera should be OK, although there are many ways things could be done better. If you don't think that is is OK, then why not? Is it the equipment or a problem with the photographer, which could certainly be the case since I was still finding my way around...

I have an extensive experience with Canon equipment. I love the SL1 and keep it with the following lenses, saving the more exotic and expensive for my larger body DSLR. I highly recommend these three lenses which can be obtained, used on Ebay, for a very reasonable price:

18-135mm EF-S STM general purpose zoom
55-250 EF-S STM telephoto
either 50mm 1.8 prime lens or 40mm pancake 2.8

These three lenses will give you the flexibility to shoot most situations with excellent image quality. They are small, lightweight, and mate beautifully with the SL1. All three can be purchased for around a total outlay of $600 or less.

Telephoto-

Canon EF 75-300mm (the current non-IS and non-USM) just isn't one of their best lenses. It's the cheapest... and it's a rather powerful telephoto on an SL-1, so basically you've already got "telephoto" covered and might just use that. Unless you really feel the need for and want a better lens... I'd recommend the EF-S 55-250mm IS STM as the most affordable at about $300.

Or if you feel that 250 to 300mm just isn't long enough... 500mm, 600mm and even larger are possible. Canon makes a lot of longer teles... as do Tamron and Sigma. But, be warned, the prices start around $1000 and go up well over $10,000... plus these lenses are bigger and heavier, so you might have the added cost of a sturdy tripod to hold the lens!

Wide angle-

Canon EF-S 10-18mm IS STM at $300 is probably the best value in a wide angle lens from anyone right now. The next most affordable lenses typically cost $150 to $200 more and don't give all that better performance.

Canon EF-S 10-22mm USM is an older model, but probably the very best ultrawide made by anyone... sharp, well corrected and very good handling flare. It costs about $600.

Macro-

One of the least expensive ways to do "macro/close-up" is get a set of Kenko Macro Extension Tubes ($110 currently... on Amazon). A year ago these were selling for $200. They're good quality and simple to use. Just install one or more of the tubes in between the camera and your standard zoom (probably an18-55mm or 18-135mm) or telephoto zoom (the 75-300mm or any other telezoom you might use), and the extension tubes will make that lens more close-focusing. More extension = higher magnification.

There are cheaper extension tubes (Opteka/Vellow, Zeikos/Bower/Vivitar) that aren't quite the quality of the Kenko (and Canon's own, which are more expensive). Those might serve fine for less frequent use. The versions of these with metal bayonet mounts seem to sell for $60-75. (There's an "economy" version of some that uses plastic mounts... I'd probably avoid those, unless only using very rarely and with small, lightweight lenses.)

DO NOT get the super cheap macro extension tubes (under $25) that don't have electronic contacts to control the lens. They can be made to work, but are a real pain and very slow to shoot with when used with any of the modern, electronically controlled lenses. I guarantee you that you'll be happier if you spend a bit more for any of the above mentioned extension tubes, with the electronic contacts to fully support the modern lenses you are using.

If you don't mind the extra cost and prefer a true macro lens - which is usually the fastest and easiest to work with, and pretty much assures very high image quality - the Canon EF-S 60mm f2.8 USM Macro lens is one of the best deals around, at about $420. The Tokina AT-X 100mm f2.8 Macro is also pretty darned good and a bit less expensive at around $350. There are a number of other macro lenses available from Canon, Tamron and Sigma.

Personally I use the Tamron SP 60mm f2.0 Macro/Portrait lens ($525), because it's quite compact and has a larger aperture than most other macro lenses, which is nice to have for portraiture, in particular. I also use the larger and heavier Canon 100/2.8 USM (about $550), and highly recommend it as a top quality, versatile choice.

Which to buy first?

Sorry, I have absolutely no idea and really can't help you with this. No one else can, either. Only you can set your priorities. What do you want to shoot most? Bugs and flowers = macro. Sports and wildlife = telephoto. Scenery and architecture = wide angle.

Do yourself a huge favor and get a couple books that will explain a lot of things about photography in more detail and far better than we can hope to do here on a blog. I highly recommend you get one of the guide books specific to your SL-1 camera... Also get copy of Bryan Peterson's "Understanding Exposure" for more general info about photography with today's cameras and lenses. These will be very helpful to new DSLR users like yourself and maybe should be required reading before posting questions on forums and blogs!

I will use the EF-S 18-55mm f/3.5-5.6 IS II as an example.
The EF-S means that it is for Canon APS-C (Advanced Photo System type-C) cameras (crop sensor cameras like the rebels and the 7D line that have small sensors)

f/3.5-5.6 means that at 18mm the lowest f/stop will be f/3.5, but as you zoom it will go up so that by the time you get to 55mm it will only be able to go as low as f/5.6

the IS means that it has Image Stabilizer. The II means that it is the second of this lens

Does this help?

The ultra-wide angle, Canon EF-S 10-18 mm is around $300. I bought that one new, but have two other lenses I purchased used (18-135 mm and 70-300 mm) that I'm very happy with, so you might consider that avenue. Good reputation companies include B&H, KEH, Adorama.

The 10-18 mm is challenging because an ulra-wide wants to include every little thing in your composition, and everything except what's immediately in front of you looks very small.

If that tweaks your interest, here is a topic with a lot of info and photos. Be sure to look through all pages because there are many that are much better than the two I posted :)

http://www.uglyhedgehog.com/t-336465-1.html