A vote here for the GForce. The rating I was quoted was 650ft/lbs of torque in a 3400# car with the upgraded main shaft. We have the upgraded GForce in my son's 89 Gt convertable, and are very pleased with it. This was even with dealer problems. (Go to GForce for the trany!). The shifting is solid and smooth. The case is not the main problem with the T5, it's stock mainshaft is. The GForce solves this with the upgraded shaft and inter re-enforement plate. Also love that .59 overdrive on the hiway with the 3.73s in the GT.
I have heard from a friend whom has the T500, that it shifts more positive than his 500. He's driven ours, but I've yet to drive his. He also had to 'clearance' the tunnel to make room for the 500, new shifter, new output slip yoke, speedo cable didn't fit. When I was researching the tranys, there was alot of posting of problems with the 500/600s. Hopefully these have been resolved as I haven't read many lately.