Yes, the 1DX suggestion was somewhat in jest. There are reasons to move up from one camera in a line to something several steps up, but I would not say that buffer speed should be one of them, unless you are doing it professionally. Since you know the activity, you can use that to make better shots. Instead of firing continuously, you could watch the movement and take a shot when the moment is right. Then the buffer isn't an issue, but shutter lag definitely is.

If you find that the T3i is getting in the way, the 70D would be a good next step, skipping the T5i. This has excellent AF tracking to keep up with moving subjects. But if you haven't used the best possible technique with the T3i, the results won't be much better with the 70D, especially for action shots. But if you can get the full potential of the equipment, the dancers will love your photos. :-)

First off May I ask why you are using a 64GB card, you shooting video? I have had experienced Card failures even with retrieval software was not able to recover images of lost data, it's been my experience to use smaller capacity cards like 16GB due to less chance of loosing the whole lot of images on one card plus they are much cheaper than one 64GB card and unless you are shooting rapid shutter releases like at a sporting event or a modeling shoot then large capacity cards are not as much needed unless you start using cameras with 16megapixels or higher, I use 2-16GB cards in my Nikon D3 and D3x and a 32GB in my D800e and I shoot weddings of normally 1500 images, but I also would rather change out a smaller card than possibly loose all images off of just one large card.

Then are you shooting JPEG or RAW? I shoot RAW so image size is much larger as a File size than a JPEG file size.

The only reason I can see buying 64GB or larger is for shooting video unless you shoot Sports and modeling professionally, otherwise it's a waist of money and not much of a gain plus it takes longer to transfer files with large capacity cards filled up and even longer if camera is tethered to computer.

I believe you only need a new card reader. The description for yours on Amazon says, "Compalible with all versions: SD/HC N,MicroSD, CF. MS. XD and Sim Cards." It was probably produced before the SDXC format was created. The card will work as fast as your computer is capable, but no faster. It's a safe bet that the Canon 1DX has the fastest buffer speed in the Canon DSLR line.

You should look for a card reader which explicitly says it supports SDXC and is USB 3.0 speed. Your computer may not do USB 3.0, but your next one might, so you won't have to get a new card reader then. (one option on Amazon)

I believe you only need a new card reader. The description for yours on Amazon says, "Compalible with all versions: SD/HC N,MicroSD, CF. MS. XD and Sim Cards." It was probably produced before the SDXC format was created. The card will work as fast as your computer is capable, but no faster. It's a safe bet that the Canon 1DX has the fastest buffer speed in the Canon DSLR line.

You should look for a card reader which explicitly says it supports SDXC and is USB 3.0 speed. Your computer may not do USB 3.0, but your next one might, so you won't have to get a new card reader then. (one option on Amazon)