I have a TM P90 (2 actually as daughter has one), you dont need to mess with them they work great out of the box. The M4 is ugh by comparison, the P90 is rock solid, supports a scary (based on M4 hi cap) box conversion and I think works best with its nice short barrel.
Personally while I dont mind the front rail stuff, I agree P90's dont need it. That very short design is their main strength and extra length wont mean more power. I play woodland with mine no issues, its great to be able to play one handed while carrying the 'Bomb' objective with the other for example.
I have had some feed issues with actual P90 mags, but none with the box conversion which also avoids the need to get P90 mag pouches. Batts are no issue, go for small low cap LIPO's and just carry a few as they are cheap. Swapping out every few games through the day. Or you can wire in the large batt in the box mag, but I prefer to keep them in the stock.