The gearing will have to change slightly, but not dramatically - when you switch to higher voltage, you need to switch the motor to match it (in most cases, but certainly not all)

you can go lower in the battery mah rating when running higher voltage aswell which will yield the same results as a lower voltage setup - but again - you will be running cooler...

the 6s setups look to be the best bet power and efficiency-wise, but 4s will do the job just fine - just lose as much weight as possible on a 4s setup - pretty sure most places run 15 minute mains anyway, so it's not that big of a deal...

i am yet to...
a) run my truggy at a track, and
b) run anything higher than 4S tho, so cannot speak too much from experience - just what i have read =)

I am hopefully gonna get a chance to run my mbx5t at a track on sunday, so will post how it goes... i originally opted for the tekin rx8 2000kv truggy combo, but wasn't thinking through the conversion, and mounted up a neu 1512 during mock up, assuming the T8 would be the same (long story as to how that happened) - i was wrong... so i'm gonna try the 1512 2.5d i have from the buggy (and swap the T8 into the buggy) in the truggy on 4s, and see how it goes - will go back to the drawing board and throw in the T8 if it doesn't go well enough...