The Silverlight-version of this game causes a CPU-load of around 70% on my laptop (dual core 2.2ghz), even if I do nothing but viewing the game screen. I have installed the newest SL Plugin on the newest Firefox. Maybe you can (or already have?) implement performance-increasing methodologies in ExEn depending on the appropiate Silverlight version in order to reduce this immense CPU-load. As long as Silverlight 5 has the XNA support already implemented, you’ll probably change the way of porting to SL5 anyway, do you?
BTW: I’ve never investigated XNA game development, but currently working on a game in Silverlight (almost done so far) which will be a remake of a C64-game called “Sensitive” (not sure if you still got to know this state-of-the-art home computer at the 80’s 😉 ). Maybe I had better to do that with XNA however. For the next game I will do that so I can port it (hopefully easily) with ExEn to other platforms. Thanks!