Yep, I've flown a plane then taped the surfaces and checked the difference. Noticeable improvement, espeically in roll. I think if your surfaces don't deflect much, the difference will be less noticeable. I honestly didn't bother on my beater-trainer, but I have it on my other planes.

I once flew my overweight Cap21 with one aileron sealed, and the other not sealed. I don't recommend this trick on an acrobatic plane, that's for sure. Not only was the roll rate noticeable faster in one direction than the other, it also seemed to affect looping and stalls. I didn't fly it that way for very long, or do an extensive testing routine, but it was enough to convince me.

Btw, I've seen several IMAC-type planes that have the surface resessed a little in to the fixed portion, or have composite layers that sit over top of the hinge area. This kind of thing gives the same sealing effect as tape. Lots of way to stop or reduce airflow through the hingelines.