It really depends on how you intend to use it and the body. I have used the 50 f/1.4 on a 400D but did not like it much. No complains on the image quality (it is great) but AF was considerably slower compared to the 100mm f/2.8 or 15-85mm f/3.5-5.6, all of which have ring USM. It was bad enough to keep causing me to miss shots of my toddler. I have now upgraded to a 7D after the LCD on the 400D was spoilt (I was looking for an excuse to upgrade anyway and have not found myself complaining of the slow AF on the 50 f/1.4 since. I did not compare it with the other 2 lenses mentioned but no longer feel a need to do so, which says something good about pairing this lens with the 7D.
A friend tried both the 50 f/1.4 and 85 f/1.8, and decided to go for the latter. He also uses the 7D. Whatever reason it is that worked for him, I would still have chosen the 50 f/1.4 for the smaller size and weight. Before you make the switch, you could rent/borrow a copy of the 85 f/1.8 and try it out to see if it suits you better. Your intention to go for "the best AF performance" might have been influenced by your experience with the T2i. Once your 7D arrives, you might have a different viewpoint on your priorities, especially if your purpose/types of subjects and lens collection also start to grow.
Keep us posted on your decision.