Provided the correct Crystal, CPU, and RAM is in place, can the XT286 go faster than 6Mhz, or does it also have those ROM routines that prevent this like some of the ATs do?
Assuming the RAM is fast enough, I don't see why an 8MHz 80286 couldn't be used. Even with the 6 MHz original 80286, the 5162 is almost as fast as an 8 MHz 5170. OC-ing on the 5170 went to ridiculous clock rates (for the time).