In my experience of other cars with a much lower power output, an increase of 7BHP would be feasible from just the exhaust! looking at it from a percentage point of view, I am talking about a 4% increase, not a lot really, for the improvements made to be felt so noticeably by my exhaust I would say it has increased the bhp output, but the biggest change is to the torque curve, however with out a rolling road test before and after you just don't know, its a guess. Most performance type air filters claim upto about 5BHP for their products...
On my 'Vette the std (very bad) exhaust and cat were removed along with the old manifolds, for a gain of around 40BHP, now the original system was cr4p, so there is more scope for improvement, plus '75 was the first year 'Vettes had Cats. Now the 156 exhaust is a better more suitiable design to start with and the whole drivetrain is pretuned by Alfa, which is why the gains will be much less.
A CDA cannot be worth pushing 200 Quid if it doesn't gain 4 or 5 Bhp, I have seen proper remaps for my engine that give 11Bhp for 400 ish, the step after that was a set of cams, and the remap for a couple of grand , giving 20 ish Bhp in total (not worth it IMHO)
I think if you want more power from the 2.5, you need to swap it for a 3.0 or 3.2, there is a guy on Ebay who will fit a Recon 3.0 for 2K if your old engine is shot, probably cheaper if your's is OK, and if its a recon it should
have a new cambelt, which is worth 400 notes making it even better value - Food for thought