I've run 13", 14" and 15" on my ITB and ITC Sciroccos. The order of my use was 13, 14, 13, 15, so I can only really compare the 14 and 15 to the 13, and not directly to each other. All wheels were 6" wide, and weighed from 8-13 lbs without tires. I've already accounted for gearing, so that can be neglected in the results.
13" with Hoosier R4/R5 225/45-13. With a MacStrut limited car like the VWs, the 13s do have the advantage of getting the car lower. The small diameter tires still available can also help with gearing for ITC. The 13s were actually not my lightest wheels, but the tire/wheel combo was lighter than the others (although significantly heavier than with the old Hoosier Street TDs). Unfortunately, both the feel and ultimate grip of this combo fell short of the others. The drag of the wider tire also seemed to be a factor at faster tracks.
14" with Goodyear GSCS in 195/60-14. Responsiveness (turn-in, etc.) was much better than the 13s, but ultimate grip seemed only slightly better (too close to call since different brands). Drag was greatly reduced at faster tracks compared to the wider 13s. These were faster than the 13s, but a bit less forgiving (surprisingly).
15" with Hoosier R5 205/50-15. Responsiveness was excellent -- I consciously had to delay turn-in compared to the 13s, and ultimate grip was even better. Lap times fell by roughly 1 sec compared to the 13s. These were actually more confidence-inspiring than the 13s, at least after they were warmed up (which did take longer).
Without a doubt, I'd definitely recommend the 205/50-15 for ITB, especially if you're currently using a 14 that's wider than 205. There's more grip, less drag, and really no down side that I can think of. It's a tougher call for ITC, but only due to gearing issues.