I'm using a Neu 1515/2Y (1100kv) since one year. It powers my
truggy (started with a Neu 1512/2Y 1400kV),
buggy/truggy hybrid,
onroad buggy and
trophy truck with 9s (27V), 10s (30V) or 11s1p (33V) A123. Very versatile motor.
I don't think you will see much improvement over your Neu 1515/2.5d. You can reach peak power (around 3KW) with 6s too. Efficiency won't be that much better so that you will run a few minutes more. Runtime mostly depends on how hard you are using throttle. If you are happy with power and runtime of your actual setup stick with it. If you want to experiment and money is no concern -> go for it.
Concerning your MGM ESC. Firmware update is free of charge (but you have to enclose some money for shipping it back and the cc03 cable which enables you to make future firmware upgrades yourself). You have to enclose your USBcom module (+cc02 cable). Best would be to send an email to Mr Dvorsky:
gdvorsky@mgm-compro.cz.