48cm would be a pretty small frame, and happens to the be the smallest size listed for the CAAD8. The Synapse apparently comes in a 44cm.

I ride what would be considered a snug fit with my bike. It has never been a problem, and I've never considered it to be a particular danger to the jewels. I have a habit of sitting on the top tube while stopped. Keep in mind, once riding, the top tube height is irrelevant.