nsr250repsol,
RCPD makes a high quality conversion kit. When you choose the Ultimate battery tray it provides a very clean installation.
I have used both a single 4s 14.8v battery and I have tried 2 x 2s 7.4v batteries (wired in series) and the single battery has a lower center of gravity due to the reduction of packaging.
My crystal ball tells me that we are on the verge of higher C rated batteries being released (between now and the spring). Until now 25c were the top discharge rate but before the spring we will see 30 and 35c become available at the same prices as the current batteries or even lower. When choosing batteries I would rather have a TRUE 4s pack which is made up of 4 battery cells wired in series as apposed to a 4s pack which is made up of 8 battery cells which are wired 2s 1P.