Digital projectors can produce a moiré pattern on the screen as the LCD/DLP arrays' line up with the perforations, but the only way to know for sure is to try before you buy if possible.
The holes will at least rob you of 10% of the light reflected back. Not sure if this can be rectified by upping the brightness, but surely that will have an adverse effect on the rest of the image?
I read somewhere that Stewart Screens supply a post processor of some sort so that you can try and get back the sound quality you lose with a perforated screen.
Some swear by them though, so maybe it's worth it. I can see that having a speaker on the floor isn't going to do the center channel many favours, but putting it on a stand and angling it up to the viewing position seems to work quite well IMHO.