I'm not sure why the USAF need it -nobody else needs that type of capability in a trainer
May I offer my two cent's worth:
Penny #1: That level of performance is almost certainly not needed for undergraduate pilot training. However, the same trainer will be used for refresher training of pilots who fly the F-22 and whatever future air superiority fighters may come along. It's really expensive to operate front line fighters and a high performance trainer can provide a big percentage of the refresher training needed to keep the pilots current in those fighters. Further, the T-X will very likely form the basis for the "red-air aggressor aircraft." That's the aircraft that will be used to simulate 5th (and maybe 6th) generation opponent aircraft. As such, it will need exceptional performance.
Penny #2: The 6.5G sustained turn requirement can be met by several existing trainers. However, most cannot meet that sustained turn performance at the altitudes USAF wants to train its pilots. So its not so much the raw turn performance that's proving difficult, but doing it at the altitude USAF demands. USAF apparently wants to do the high G stuff at higher altitudes to provide a greater margin of safety to the pilots being trained.
Hope this was helpful.