I have the OS 160 with slimline pitts, 2 225's in the tail, 945mg in the bay, and 6v nimh in the engine box. No added weight and cg is at 4.75"
Aft CG means pitch sensitive, not unflyable. Decrease the elevator throws. Remcl, don't think that will cure your prob though, your wing loading is probably too high to be stable with an aft CG.