ORIGINAL: willflyforfood
The ailerons go down more than up.
There's your problem.
The nylon "ears" used on 80% of the aileron installations out there tend to put the attach point of the pushrod ahead of the aileron hinge line, which effectively gives a little bit of forward rake on the horn. (I know it's a wire with a nylon ear on it, but think of it as a horn.) On a low-wing airplane, that's exactly what you want. If the servo arm is straight across, that little bit of rake on the horn causes the servo to pull the aileron that it's pulling just a few degrees more than it pushes the aileron it's pushing.
BUT ... on a high-wing airplane like most of today's Q500 racers, that same angular difference gives more down aileron than up. The solution is either to bend both horns back a few degrees, or (as you suggest) use a round wheel on the servo and drill your pushrod holes a few degrees ahead of the center. Now you have the same rake at the servo as you have at the horn, and the aileron travel will be the same amount both up and down. Season to taste. [8D]