Sounds like its too nose heavy. I'd try to move the engine further back on the fuse and maybe the battery. With a further aft CG the tail should come alive.

I've been beating my 6lb TT(thunder tank)Fun Tiger with a 46fx and MAC muffler. It revs faster and seems to turn more R's with the 11x5 zinger, but it does climb better with a 12.25x3.75 APC.