There are many quality servos that don't need the higher voltage. That being said, maybe a LiFe receiver pack is what you are looking for. They come in all shapes and sizes as well. A two cell LiFe pack puts out 6.6v, and if you must use HV servos, then I guess LiPo would be something to look into.
I'm not into nitro side of the hobby and don't follow the "ins and outs" of it. I do know 1/8 scale truggy drivers use either NiMh, LiPo, or LiFe packs in their rides. The debate has been going on for years; what to use for a receiver pack. Maybe check out/post in the nitro forum?...I see you already have.
