A goal in a true Pattern plane is to achieve as close to total neutral stability on the pitch and roll axis as possible (not easy to do) and is sometimes called grovin or 'goes where you point it' as opposed to the high positive pitch and roll stability that is designed into a trainer and called 'self recovering' or self righting.

Removing the dihedral on a trainer will definately give more of that grovin type of roll character and is a lot more fun, in some ways easier to fly that way. However even with the wing totaly flat, it will still not be as close to a neutral stability in roll as a pattern ship and the reason is with a high wing cabin type even with a flat wing will still experiance a 'righting tendency' due the pendelum effect of the wing position or more properly a dihedral effect.

I think you will find it interesting if you are going to flatten a trainer wing anyway don,t stop there build in anhedral or drooping wings. Sounds crazy and looks silly but you will amaze yourself and your Buds how much better it will fly. Actually a low wing pattern ship needs a very slight amount of dihedral to reach that neutral point just the opposite of the high wing, or it may have a subtle taper in airfoil towards tip and the tips as well may be providing that needed dihedral.

I have 'bashed a few this way for friends and the results are delightful.

John