I ran a 3.5 when I was looking at single cell LiPo. It's pretty fast but drives different than NiMh with a 4.5 turn. Even at legal weight.
I agree that the new NiMh cells are way better than what we had about a year ago. Much less maintenance and they're lasting longer. That being said, I'll be trying more LiPo stuff this winter.
Changes to the classes will come as people try new things at the club level. Not so much from the big races.