To back up SesameCrunch: the Capreo rear mech definitely wraps the chain further around the sprocket. When using a Capreo cassette/hub with a Tiagra rear mech, we had terrible problems with it skipping in the 9/10 tooth sprockets at the back. This was on a tandem with 20" wheels, though I could cause the skipping when riding it solo. The Capreo mech fixed the skipping as soon as they swapped it on.

We got the new (at the time) long cage version, and it's still specced as having 15 tooth capacity (the spec is identical to the short cage version, so clearly it's conservative and I suspect based on the 9-26 cassette). It still claims to be designed for a single front ring, though we're happily using it with 38/53 up front. I avoid the 38-9 combination, and the 53-26, though they both do work with a little noise from the transmission...

I think, though I wouldn't swear to it, even the Capreo with the slightly longer cage gives a bit more ground clearance than an equivalent "normal" derailleur - a significant plus on a small-wheeled bike!