Countering a wing warp with a trim tab is kinda like letting the air out of your car's left front tire when the right front tire goes flat. I did a search on the Banshee on the Web. It looks like the LE isn't sheeted, so the structure can be dewarped easily. Look at the wing from behind. Hold it so the TE is centered on the wing thickness. You should be able to see the warp.
If the airplane is covered with plastic, heat the covering with a heat gun or iron wile you bend the wing opposite the warp. Keep holding in the twist until the wing cools. If it's covered with paper or silk, pour boiling water on the wing while you bend the wing opposite the warp. Keep holding in the twist until the wing cools. The purpose of the towel is to mop up the water. You don't need to put it on the airplane.
You can dewarp flaps using the same methods. If your problem is misaligned flaps, rather than warped flaps or wings, you should tweak the flap control horn. You can put pieces of plywood on either side of each flap where the horn arm is and usually bend the horn without hurting the flaps, as Brett says.
Standard field equipment for combat guys, who are always trimming new airplanes (combat planes don't last), is a Monokote iron and generator or inverter for dewarping.