Which 400mm Mark II? Canon makes two: EF 400/2.8L II IS USM and a 400/4 DO II IS USM.

I don't have either lens... The "Mark III" teleconverters supposedly works slightly better on the f2.8 lens, than on the DO. But the latter is still very good.

Compare either lens with and without 1.4X and 2X, and against each other for yourself: http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=962&Camera=453&Sample=0&FLI=1&API=2&LensComp=962&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=0

Optically, there doesn't seem to be a whole lot of difference between the Canon 1.4X II and 1.4X III.

The 2X III was a bigger improvement over earlier versions in terms of image quality (but IMO the II really works quite well on some lenses).

Something I found a bit surprising..... When first introduced Canon's announcement about the III teleconverters mentioned that the new extenders' electronics were designed to slow down auto focus by about 25%. Maybe this was to promote accuracy, since the lens + extender would result in shallower depth of field.

I use both 1.4X II and 2X II. The latter I mostly just use on 300/2.8 and 500/4 primes, where it works great. The 1.4X I use a lot more often, with those same lenses as well as a number of others incl. 135/2, 300/4 and 70-200. Recently got the 100-400 II and plan to experiment using the 1.4X II on it, too.

I have no plans to "upgrade" to the 1.4X III, because I just don't see much or any difference compared to the II version.

While it works fine on my 300 and 500mm primes, I definitely won't use the 2X II on any zooms. Haven't found a zoom + 2X II combo that gives me images I'm happy with. Some people feel the EF 70-200/2.8L IS "Mark II" with the 2X III works very well... (much better than the combo of the earlier versions of each). But I wouldn't expect it to come close to the quality of a 400mm prime and won't be running out to buy it because other lenses in my kit handle the focal length.

Yes! Most of the birders that I shoot with are using a 1.4 or a 2X TC on everything they shoot. A 2X with a 300mm or a 1.4 on a 600mm prime with a full frame works very well. You have to be careful when doing all of this on a crop body as too much magnification because soon you will feel that you are trying to find your subject while looking through a straw. Birds can and do fly very fast and erratic. I do 99% of my BIF off a tripod with a jobu gimbal head. Do yourself a big favor and try some BIF and you will very quickly see where the problem is and it's good practice for your surfing shots too. Have you micro adjusted your body to your lens? It may not make any difference if you are shooting stopped down. All three of my Canon TC are series III.

For finding a subject without the "looking through a straw" feel try using a Red Dot sight on the hotshoe. You do have to carefully adjust/align the center point with the dot at a picked distance and then learn how much "kentucky windage" to use at other distances. You can afford a lot of shots to practice/learn with digital. But being able to use both eyes to find a subject, or track a moving subject really helps. And I often use this setup with a Nest Gimbal Head on a tripod. When hand holding I use a pistol grip attached to the tripod collar. The tripod and gimbal work best for fast or erratic subjects while the hand held does just as well on stationary or straight line movement.

I ran into that problem with birds in dim light and the need for flash(sometimes with a better beamer) so I looked around and found a hotshoe splitter on ebay. I am now experimenting with that. When I got my two (two bodies) there was only one seller and brand, but now there are several. There is one seller with a Red Dot and adapter combo for sale as a bundle but I prefer a different Red Dot (lots of them on Amazon, same style I use with several brand names, probably from the same factory in China) and a different style adapter. MT Shooter has a good adapter with extra length for eye relief and balance adjustment on his Camera Cottage site. I have one of them and one simple little one I got a couple of years ago.
Here is what that rig looks like. Ignore our son's HO layout, the table was handy for doing pics of the rig.