I thought of that. Two reasons I guess. I would have to buy one, and I am concerned about the same issues in 1 above....image degradation and vignetting when used in conjunction w/ the yellow filter. I really like the K2 filter to add contrast and punch up the skies a little.
I was thinking of going out and shooting tomorrow, so it would help if I can get by w/ what I have, as I would have to order a ND filter.