I progressed at moving over to using Farseer for all the game objects. I re-factored the Growth object, moving all the Farseer specific stuff into the base object that the projectiles also share. After a bit of debugging, I was able to get the projectile created and moving properly.
Next steps include porting the player controlled avatar to use Farseer. I need to disable and discard the old collision detection routines and then use the detection that Farseer is already doing. After that, I need to tweak some settings, since I notice significant slowdowns with a large number of items. Though, that may be reduced once the old collision routine is disabled.