I am looking for something that will be able to get these shots in focus, have better low light focusing, and possibly better low light performance.Low light AF of stationary subjects is very different to low light AF of moving subjects. For that reason, I'd stay clear of anything which can't keep up with very rapid moving subjects, which means forget the X-E1 and the Sigma 50/1.4 as they'll both give worse performance that your current kit. (I have a good copy of the Sigma 50/1.4 which AF's very accurately. However, AF is slow, and in AI Servo mode, just forget it)
I have considered the following:
Don't do anything at all! (Parents would love this! )
Purchase a few fast primes. (Sigma 50mm f1.4)
Buy a Fujifilm X-E1 due to it's size and low light performance. Then down the road, buy a 7D Mark II or some "upgrade" to my T2i. <--- Most money spent here
Buy a Canon 70D during black friday. (No tax day at two of my local camera stores)
Wait for a 7D replacement
I'd recommend a fast focusing lens of at least f2.8 - if you stick with crop, the Canon 17-55/2.8 IS springs to mind, or the 24-70 II if you have money to burn and something like a 5D3 might be on the horizon soon. Both give very fast and accurate AF, even in challenging conditions. If you're after a fast prime, steer clear of slow focusing lenses like the Sigma 50/1.4 or 85L.
As for the body, the 70D with what appears to be an inherited 7D AF system should be a huge step up from your 550D. While rumours suggest the 7D mk II will have an even better AF system, that is largely an unknown at the moment. However, a camera with a good AF system still can't operate at its best with an f4 lens or with a painfully slow focusing lens such as the Sigma 50/1.4.
If a 7D mk II is potentially just about affordable for you, what about instead getting a 70D and 17-55? The combination of lens and body will bring big benefits to you.