I don't want to argue about anything in the above post's HOWEVER there is something in your original post no one has commented on.
Do the foam ailerons flex the same from the inner to the outer end? you need to measure this and see what the diference is.
Does the aileron servo stall at full deflection.
my point here is sometimes foam ailerons use the foam as a hinge and it does not bend well away from the servo horn and this could also cause your plane not to roll well.
I have seen the guy's at rcsuperpower's fly this plane on youtube it really looked like they were having fun with it. I almost bought one myself.

There is a guy at our field who taught himsef how to fly by flying low cost foam planes last time I saw him he had a brand new bixler glider from hobbyking with duct tape all over it he used duct tape for the hinge line on the ailerons I don't recomend this Blenderm is the stuff most people use for hinges on foam planes it is available at drugstores in the first aid aisle. Greg