8.5 was a common motor at a very tight 50 x 50 track if that by me. NOT because people were looking for insane speed, but the torque so you could punch over the short jumps.

i could win the class using a 17.5 depending on how the track was setup. if flowing, could do using boost, etc. But, if not, not enough punch.

Now, that said, personally I think a 10.5 boosted on a lot of tracks is probably plenty.

As for 8.5 versus 7.5, I haven't seen or heard of anything going this kind of switch made the difference for them.