I have 36 at the moment, and they are kept full of steam. 36 is the exact equivalent of RC Industrial or Commercial Steam Engines, so TE3 is no *less* efficient at converting steam to RF or MJ than Railcraft.
I have not, however, tried to put more than that on it. TE3 Dynamos *may* be more efficient, but I wouldn't bet on it. KL and CoFH try very hard not to step on other mod's toes.