On my TF Corsair, stock is 0o wing incidence. I've done a suggested mod to 2o+ incidence. Putting the wing on the plane, I find that the port wing is indeed at 2o+. However, the starboard wing is 3o+ (that 3 piece wing is a son of a gun to get perfect).

Is that enough of a difference to cause flight problems?

If it is, how should it be corrected?

If it was not enough, that would not be an issue (just put a spacer in the rear bolt area). However, as it's too much, I can't just shave down the saddle (or can I)??

Hi!
Yes!

As you have diffrent angles it means that one wing panlel is flying at a higher angle of attac than the other...which is no good (means instant crasch).

I fixed this many times on my friends airplanes and what you do is to slice up the wing surface and then twist the wing and at the same fill in the balsa surface with CA glue so the wing retains its new angle.