Well, did not try the 11.5x4 but removed the exhaust deflector I had on the muffler, and look and see, the performance improved ! It is not quite where I wanted it, but much better, and I think I can leave the engine on for now. It does now climb straight up out of a hover (with the APC 12x4) ! I think that the deflector was putting on to much back pressure.

Jeff, if you have not flown any 3D plane before, be gentle and careful with the Flip. It is not a beginners 3D plane. It can be very wild, and it DOES STALL (actually cool for some wild maneuvers)