I had a 2010 Prius V (the one with the 17 inch wheels) and at first I got about 5 mpg less in the CT200h, but I was willing to accept that since the luxury features and accessories of the Lexus (not to mention the dealer experience) were so much better. I now have about 4,000 miles on the odo now and it has crept back up to where the numbers are really very similar. I think comparing mpg figures from internet posters is ludicrous. Some are painfully honest, some are hard to believe, some are ridiculous. Comparing my prius figures with my CT200 figures, I am now at about 45-48 mpg, which is just about where I was with my prius. It got a lot better the last two tankfuls.
In short, you might lose a couple of mpg. IMO that is well worth it.