I have a couple of PT 40s, here is what I would do:

1) Increase the throw - if that does not work:
2) mount individual servos for the alierons, this will help increase the throws - if that does not work;
3) take the wing apart to take the dihedral angle out - this is some work, but will make the plane fly much better. If that does not work
4) buy a new wing kit and build it without the dihedral, you will be happy you did - more work and more money, but the best overall solution.

I hated the dihedral in my first PT-40 wing, I wanted to replace it, and went to get a wing kit, I found out a new full kit was $15 more, so I bought the full kit and rebuilt everything (much better the second time BTW). I met a guy at my club who burried his PT-40, killing the fuse, but the wing was fine, he gave it to me, so I converted the first one to a tail dragger and I have a spare wing that I will never use because of the dihedral.

BTW - they are trainers, so even now with a lot of throw, high rates, dual aileron servos and the low dihedral wing and it still has trouble with doing a roll.