I only stripped the underseal off the inside of 1 wing and it's pretty much as you describe! Only the bottom of the wing is painted, the top (if the wing is bolted to the car) has a light dusting of paint.
I wouldn't paint it, it's only ever original once and that cannot be replaced nor replicated. I wouldn't worry about concours points, if they wish to drop you points for having completely original paint even on the inside of the wing against someone who has had to repaint the inside of their wing then it's a hit I'd rather take than to lose original features.

