First off: Are these very dusty races? Just curious, you mentioned you were bringing 2 extra filters. Sounds like you are following the owners manual...this may be overkill. In reality, you should either:
1. Never change the filter, but change pistons every race
2. Never change the piston, but clean the filter constantly
ok, enough humor, seriously I think 3 filters on race day is a bit much, but that's just my opinion. Seeing some dirt on the outside of the filter is ok, it's doing it's job. As long as it doesn't make it thru to the inside (which takes a while) you are ok. The only time I have seen evidence of dirt making it through to the inside of a filter was when either it was forced through with a powerwasher, or the filter wasn't oiled properly. I have seen filters that looked like a big ball of dirt on the outside, while still clean on the inside (not that I condone neglecting maintenance).
I was wondering the same thing about richening the mixure (restricting air flow) by using the skins. On my 250 I haven't noticed much if any richening, even when the skin is pretty coated with dust. Another option is to not coat the skin with oil. You will have to clean the filter more often, but it seems to make a protective layer on the outside that the dirt can "bounce off of" rather than "stick to". An oiled filter or skin really sucks in the dust and doesn't let it go. So I think this is a good compromise to leave the skin dry. If you just oiled your filter, the skin is going to suck up some of the oil unless you let the filter dry for awhile.