There's a Bontrager Race X Lite Carbon Aero Track rear (tub) at my LBS. They want $299 for it. It's a pretty deep-section aero rim, with minimal spokes. It's fix/free. I believe cartridge bearings? I didn't ask, just a guess.
Any experiences with these? I do TT (short) and sprints, but I'm thinking about skipping all of that (I'm too scrawny to be a sprinter!) and going to pursuits. Don't know if that will sway your advice.
On the other hand, I could probably pick up a used disc for that price (or an old 7spd for less?).