rolf, I agree that the foils are those parts to worry about. those are small in size and deal with very specific function. the hulls in contrast are quite large and much less sensitive to the loss of flow. significant damages you probably will notice but scratches...?
just imagine the loss when pulling once too hard on the tiller, taking a wave wrong, having your sail slightly not trimmed perfectly... there is so much more to loose than with the little scratches on the hulls...

I heard once the story that the A Mitch Booth became worldchampion with was build with a 2cm difference between the two hulls as they took the first hull as mould for the second... In contrast Boyer did not like the moulds of the German Flyer when they started to build the MK 5 and AUS Flyer as they were not plain and smooth enough...

But remember all those events you have to put sticker on your hulls and they start to fall apart with some boat in the middle of the race? Now, one would assume that these causes noticeable drag, still you find the top boats encountering this problem and still finishing first...

Saying this, there is so much to win and loose in good versus bad sailing, but the smoothness of the hulls is really a minor. If you are an olympic sailor and on top of your game, you surely also don't want this <0.1 % possible lack of performance but for the rest of us I think we still deal with 10% losses by simply sailing badly from time to time... ;-)