My limited experience with my setup is this:
I have some BNC to Cat5 BalUns that allow me to run Cat5 between my batwings and receivers, rather than trying to hassle with RG8. I don't know the exact measurements, but in real world use I seem to get about the same signal loss over a 100' run as with the RG8, but the cable takes up about a third of the space and weighs next to nothing in comparison.
They look just like the units in this auction
http://cgi.ebay.com/4-mini-BNC-video-balun-Coax-CAT5-f-cable-cctv-camera-S_W0QQitemZ370085265362QQihZ024QQcategoryZ48637QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
but were built for 50 ohm, rather than 75. Some of the mixers I assist have been skeptical at first, but I've provided them with good signal...and that's what really counts, right?
-Hunter