One of the issues that I was having with the Silverlight Spectrum emulator in Beta 2 was the performance.
Sometimes it would be okay, sometimes it would be really jerky. There was definitely issues to do with what was going on in your machine and other processes.
With RC0 they introduced the CompositionTarget.Rendering event, which is an event that occurs just before rendering of the UI, which makes this much better to use for scenarios such as Game Loops or for rendering of emulators (which is very similar to a game loop). This is a much better method than using a Timer or a Storyboard.
To be honest even without this feature the performance feels so much better. Although I'm not a game writer in any shape or form (not that game writers have particular shapes), my experience with from writing the Silverlight version of the Spectrum Emulator and with my general Silverlight experience I can tell this will be a great platform for writing casual games :)