I have found a need for an 8x ND filter. Any one have a comment on the Hoya line being sold on ebay? I have a funny feeling about putting a $30.00 filter on a $1000.00 lens..........
No-one I know who has ever done serious filter testing -- and this includes better experimentalists than I, such as Ctein -- has ever found any detectable loss of on-the-film resolution with any half-decent filter, including plastic (sorry, optical resin). I have several lenses that cost well over $1000 (eg 75/2 Summicron for Leica, 38/4.5 Biogon for Alpa) and I just don't worry about what filters I use.
How far you trust e-b sellers is another matter.
I think it depends on what sort of tests you do. A resolution test might only verify that most modern filters are sufficiently flat.
When I've tested filters, I've found that in a flare-inducing situation (high contrast scene, light source in the frame), that, unsurprisingly, multicoated are better at suppressing flare and ghosting than single-coated are better than resin are better than uncoated glass. Even a cheap Vivitar VMC multicoated filter is pretty good as a filter.
Aside from the original question, there is a real difference among UV filters in how much UV they filter. Bob Atkins has a pretty good article on this on photo.net.
Other issues are mechanical--B+W and Heliopan have brass rings that don't bind easily, and Heliopan filters now come standard in slim rings, so they're better for wide lenses.