Flashing to Ultra you mean? I wouldn't recommend it.
Uhh and why not? It's perfectly normal and a lot of people do it. However after getting info at Xtremesystems, I see there are Ultra users flashing to a GT bios but simply raising the VGPU. Appearantly the GT's BIOS has faster GDDR3 timings so 1GHz memory on a GT is actually faster than 1GHz memory on the Ultra (yes I'm aware the ultra runs at 1.1GHz, this was just an example)
So you should just leave it with the GT BIOS unless you just really want to see it say ULTRA in the description screen.