I have a Great Planes .40 Piper Cub with a .46 LA with a Pitts muffler. I dont see whats this whole idea about the LA's being weak, because if properly tuned and broken-in, it will give out gobs of power. I fly my 76 in. Cub at around 1/4-1/3 throttle, and can get almost a half-hour of flying with a 12 oz. tank. Since you mentioned a slow-flying plane, the Cub is great, yet it can still teach you new things. If anything, you mentioned you might want to get into 3D, and the use of rudder is mandatory in 3D. The Cub needs you to use rudder a lot, so that will help you learn. Also, it can improve your taildragger landing and take-off skills. Add to that that it can be afun airplane to throw into a loop or wing-over, and you have a pretty sweet combo....
I'm having a blast with mine.

Here it is in an L-4 Grasshopper military scheme....those D-Day stripes really help the visibility on landing when its in front of some trees! It was my second plane, and the build was a cinch too!