Flow over the top surface of a wing is almost irrelevant. All the work is being done by the low pressure side. I've seen tunnel tests when something like a small tool or pen is left on the top surface by accident and it didn't show in the results didn't even move much. On the other hand I've seen flow separation on the underside caused by decals. You wouldn't believe me if I told you how fast the air exits off a high downforce wing t/e.