The simplest way to see the impact, if any, a UV filter has is to try it. On degradations, I have in the past done a pixel peeping test with what I think was a mid range Hoya HMC. Basically I couldn't see any difference at picture level. At pixel peeping level, there *might* have been a slightly better sharpness to the unfiltered one, but we're talking borderline perceptions here and even a slight difference in focus might account for it. In practice, no real world negative impact. In night shooting, I have seen the classic inverted ghost caused by front filters, cheap or expensive. In those conditions, no filter is the way to go.
I haven't tested filters for possible improvement due to UV rejection, but I will say recently I got a narrowband 393nm-ish filter, which is located kinda borderline between visible and UV. I can tell you the camera sensitivity at that wavelength, with all filters over the sensor removed, was very low. It would likely be even lower for an unmodified camera. Compared to possible IR contamination, UV is nothing.
I haven't tested filters for possible improvement due to UV rejection, but I will say recently I got a narrowband 393nm-ish filter, which is located kinda borderline between visible and UV. I can tell you the camera sensitivity at that wavelength, with all filters over the sensor removed, was very low. It would likely be even lower for an unmodified camera. Compared to possible IR contamination, UV is nothing.
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.