I picked up some real good advice from folks on here when I bought my cheapo, carbon clinchers from ebay recently. Thought i'd report back, having ridden them a meaningful number of times.
I'm still a little freaked by how much hassle and effort it was to fit the innertubes, including multiple punctures, but the speed impact has been really meaningful. I think it's putting between 1 & 2 miles an hour on my average. I'm growing to expect to see 20+ mph whenever I look at the speedo, which i never used to. I'm not finding the handling especially hairy either.
So assuming I don't puncture, these could be the best £240 of tri budget I've spent!