Any and all filters will degrade lens optical performance very, very slightly. It does pay to buy the most expensive filter you can afford. Too many people make the sad mistake of slapping a $5.00 bottle top on a $3,000 apo lens and wonder how they could manage to "waste so much money on that lens!". Modern filter spectral coatings have reduced light transmission lost to around 1%, sometimes as low as 0.3%. None of this of course is visible in the end result.

If a lens has worn coating, scratches, fungi or whatnot, it's not a given that any particular filter will improve the optical performance; it can't: a filter is just there to alter the characteristic of the light entering the lens. A damaged lens may well produce much more flare with a filter than without because a filter is an additional layer of glass and thus an extra reflective surface.