If he wants epic run times get a matched pair of 2S lipos and run them as 2s2p (parallel).

Now the issues he will face there while still keeping the voltage in spec would be that epic run times build epic heat and a lot more weight. It would be the same as running back to back packs without stopping at the very least.

If he wants to be "efficient" and fast and all that maybe he should try to match this 3S idea up to a 3000KV Pro4 HD. It is made to be happy with 3S, will tolerate a longer run time than the 4300 and have about the same end result top speed (a touch more actually) than the 4300 did no 2S.

In any case if he does make a habit of running that 4300 on 3S you will be in for one heck of show.. I bet the car tops over 60MHP before one of three things happens:

1 - rotor explodes in the motor
2 - fire or at least smoke
3 - a rapid diss-assembly style wipe out

In any case the show will be worth having on video.