Where you get on and off the throttle is very important to get tight waterfalls. The CG needs to be pretty far back and if you can measure the throw in degrees it should be at least 45 degs. I would think you have that much if you are maxed out because my AeroWorks Extra 260 60/90 size will get that much. It does great waterfalls as does my nephews AW Extra 260 50cc. All of the AW stuff I have been around will do good WF's so IMHO it's just a matter of getting the setup and technique right. Good luck.

Edited to add: I forgot the mention that it sounds like your CG is still too far forward if you are having trouble getting it to do a decent Elevator. The elevator surface needs to be able to hit 45 degs for that too and that's where it should be as you are descending. Full up and just enough throttle to keep the plane level in pitch. Remember as you move the CG aft it's gonna get harder to land.