well, i have spoken 2 a corally team driver, that has been doing 12th longer than he can remember, and he said this:
"with 6 cells, we had just got everything right, we could gear our motors right, our batteries lasted and everything was gr8, but that ment that we could lay down alot more power, and so rear grip was lost, the love to 4 cell promoted closer racing and concentrated more on driver skill to last the 8 mins, as opoosed to kainin the s**t out of the car for 4 mins, and having no raer grip"
If you ask me 4 cell is alot better, you dont have too much initial punch to scare yourself, and it meens that it promotes more smoother, tighter lines with more corner speed (lighter) and makes you drive like a pro.
then when you switch back to your t/c in the summer, you drive alot better, and you dont need the best batteries to make you fast, (altho it helps! lol)
just my 2 pence