I think the problem with FFP in regards to City is that their owners can pump 400m into the club and call it "sponsorship", even though it's from the same pocket - for example, the naming rights of Eastlands. Etihad Airways, also owned by (Or from the same company/family) Sheihk Mansour.
Who's to say that when FFP comes in, City won't just announce a deal for £500m with "United Arab Biscuits" to sponsor their half-time snacks? That is an advantage that they have over other clubs, save possibly Madrid and Barca.
Sponsorships etc. have to be real market value, and non-affiliated with the owner.