here are some BITMAPS (BMP) screenshots i took, to compare 3 different qualities of high post-processings (90, 95 and 100) and the FPS i had while taking these screenshots. running UT3 demo in 1024x768 full-screen by the way. I zipped the BMP files so they won't lose the details i want to show here.
to wrap this in a few words: 90 and 95 looks slightly "blurry", and when it's at 100, everything around look crisp.
so if you use a LCD monitor and running UT3 in a smaller resolution than your native LCD resolution, first thing is the normal pixel-squishing from the LCD monitor, and second thing is the added "blurriness" in 90 and 95 post-processing, so playing UT3 Windowed might be a good thing to remove the LCD "pixel-squishing" if you play at a lower-than-native resolution, and having the post-processing around 90-95.
the "crispness" can be clearly seen in the VCTF Suspense screenshots i took, the foliage far away between the bridge and the house on the left, it look crisp at 100, but not at 90 nor 95.
so if you want some more FPS, playing windowed with 90 or 95 post-processing can be a good idea. Or if you can live with the added "pixels-squishing" of the LCD monitor and the under-100 post-processing small blurriness, it's your choice.