I think Spaceclam has the best approach and basically what I would recommend, so I'll amplify a little.

Trainers can do some awesome stuff, even with the dihedral. When I get a student that claims they have out grown their trainer (typically about ten flights after a solo), I ask if I can fly their plane a second. I then fly a rolling circle with their trainer... typically their mouth hangs open a few seconds as they try to figure out how I just did that with their trainer.

Point is, Trainers can be very aerobatic... even stock trainers.

Removing the dihedral will help some issues, but if you have a computer radio you can mix out the dihedral effects. Slave same side rudder to aileron (if you move left aileron, rudder moves left too) at about 30% to start. Then slave opposite aileron to rudder (left rudder causes right aileron movement) at about 30%. You can also crank up your throws. Find a good local pilot to help you set up the mixes.

Changing the ailerons, elevator and rudder as Spaceclam described will do more for you than removing the dihedral. But if you decide to start cutting the plane, you might as well remove the dihedral as you replace the control surface.

Cheers