I don't personally have a Windows Phone 7 device, so I couldn't tell you personally about the performance using this engine.
Why are you interested in Farseer? Your game looks like it's behaving fine on whatever you're using. Are you concerned about
the performance of the engine you're using? Are you concerned about the engine's community, and how active it is?
Going from one engine to the next is challenging and time-consuming, so it may not be worth it if you don't have to. While this
engine is organized, it does have a learning curve. If you do choose to use farseer though, I'm sure you'll be able to pick it up.
Farseer's got a very helpful community =)
As far as your game goes, I noticed that the landscape has lots of jagged edges and the like. Using Farseer might be
challenging for something like this. Each Fixture's shape should not have any concave surfaces, at least based on the Farseer
I was last well-acquainted with. This would mean lots and lots of compound fixtures.