It is hard to get more power from that 4.3 unit, but then it's not exactly cutting egde tech like the F430 / Maser unit. A decent exhaust, manifold, remap to 98RON and race cats will get you up to 420bhp and that really is it. And that's still a very good specific output for an old Jag engine, it made 300bhp in it's former life (unless supercharged)! The 4.7 is a different animal all together, shedloads of torque and an achievable 470bhp with mods. The interesting thing with Aston tuners is they don't exaggerate, I think many other do.
I actually had my re-map for mid-range punch, which when you have 6 litres on tap can be insane. I very rarely see 7000rpm on my car, usually shift at 5500rpm or so which is stil rocket ship fast but still smooth. Changes at 7k are brutal and the road network isn't big enough!
We've all come to expect 100bhp/litre or more which is outrageous in a normally aspirated engine for a road appilication, not too long ago the only cars that had that were the McLaren F1 and the BMW M3, it was extremely rare. Times have moved on and Aston's tech is out of date, a very interestng thread on Pistonheads about it.