Good decision, I am the same height as you and I ride at least a 52cm. So It sounds like you need to keep you racks, etc so you can commute, but you could look at different tires and perhaps a different wheel set to get a bit more speed out of the current bike.
A nice supple, folding tire can really boost your rolling speed. Here's a blog post w/ several links about tires, size and speed that may be helpful.