Bryce 7 by my tests renders roughly 50% faster than 6.1, and a full 80% faster than 6.3, which was actually a tad slower than 6.1. Bryce 7 is a major improvement just in render speeds alone. It even allows you to create an hdri from the sky used in the scene!

Yes, Bryce 7 will install over 6.3, so be carefula to secure your User presets before reinstalling.