I compose for a living, among other things. The Symphony system would probably be great for you guys because: you compose on Logic, it's probably going to sound very good because it's Apogee (I never heard it), and I believe it currently has the lowest latency - very important for your ITB VI's.
But about the I/O's, I know it
seems like you need just 2, but you're probably going to wish you had more at some point, probably while in the heat of a new song/composition.
It's for that and other reasons that I'm not quite ready to give up my console.