The shop I got my React form said to me that they are using a 5 cell nimh batteries 2700mah for React which enhances the life of the radio usage - but is it safe they said they've been using this scheme for over 2years without any issues

I tried a lipo with a cc bec set on 6.3v for a race day and it didn't fry. Some of the threads from the nomadio forums said 5v was pushing it. I ended having the local batteries plus make me a couple 2700 4cell nimh packs, they were around $15 a piece.

If you are talking about the transciever battery, then yes, the 5 cell 2700 mah pack will work great. The transmitter on the other hand is designed to operate with a 4 cell 4.8v pack. There is no advantage, percieved or real, to using a higher voltage pack in the transmitter.

I beleive if the radio is properly voltage regulated ie can withstand a higher voltage and regulate it the battery will in turn serve more theoritically

I just use the Ballistic 2700 4 cell and it lasts the whole weekend...

It's good enought for how i drive