I also have a kit-built LT-40 and agree that something is very badly wrong with yours. My throws at high rates are well beyond those recommended and I am running an OS 46FX. Even at full throttle the plane will turn as tight a loop as you could want and there are no control problems. In fact one of my favorite maneuvers with it is to roll the plane while doing a big loop!

1) Run some locking nuts down on top of the plastic connectors on the aileron control wires to keep them from spinning around on the threads under load, 2) make sure the covering is good and tight since this is what gives the wings their strength on this plane and 3) check your servo connections and linkages.

If I were a betting man I would say that #1 above will fix it. I have experienced problems with aileron connectors spinning on the threads and the symptoms are very similar to what you are describing. Now I always put locking nuts on top as part of the build.