Try calculating the amount of alpha when the typical pattern model is flying in level flight. It's somewhere between 1/4 to 1/2 of a degree, a very small amount. So upright vs. inverted differences are also very small.

Differential is used more to get an aerobatic model to roll around it's longitudinal axis, which goes through the placement of the cg. The placement of the wing to the vertical cg is the most significant factor in the amount of differential needed to get the model to roll axially. The lower the wing, the more differential is needed.

Also, with the proliferation of separate servos driving each aileron, it is extremely important that the throws of each aileron are measured and adjusted accurately, within .1 degree. I've found anything less will effect the quality of the roll characteristics.