Apex,
What about 13.5/4 cell? I don't think Lipo is going to work against a 4-cell especially some of the higher ampdraw(25c-30c) batteries. Also I really doubt the 17.5 or 21.5 are going to supplant the 13.5 as I don't see either of them having the go on a road course which would then put people in a position to have a motor for one car in one class. Never a good scenario. We've been running TCs with 27t/13.5 NiMh/LiPo and they seem to run pretty close. I have to think at lower voltages they would be even closer.