I flew a PT 40 for years. I opted for the aileron version, which had less dihedral that the rudder-only version. If they are rudder, only, you must have the extra dihedral to turn.
If aileron version, the stated dihedral is needed for stability. The things you are saying are exactly what I experienced when I re-built the wing for very little dihedral. By that time, I was not flying the model and was a more experienced flier with several other higher performance airplanes.
One thing to check. The ailerons cause significant adverse yaw. IF you add some aileron differential, it will improve the situation. To sol;ve the situation, add the diff and learn to coordinate rudder and aileron.
Biggest lesson, any expert who tells you to modify a proven design is mostly not an expert, except in his own mind. There are mods to make on all kits to improve performance, depending on your def of performance. For a trainer, it is controllability and stability. You expert can show you how to add the differential. It is unequal movement of the ailerons, with less down than up. It is the down aileron that causes yaw in the direction of the down elevator, which is in the opposite direction of the turn. You cause it by moving the linkages above the centerline of the servo(s) such that an up elevator moves towards and across the servo axis at right angles to the linkage, and the down aileron moves away from that axis. IIRC, that was explained in the instructions.
Good luck,
Mike