It isn't just BHP figures, it's the actual drive & feel of the car.
The BHP could in theory be exactly the same, but on a custom map, you can choose when the power is delivered & by how much.
For the sake of a few lbs/ft and BHP over a 200-300 RPM difference (at the most at the top and bottom end), I think it'd be more down to the imagination of the driver/owner. They may lower the turbo coming on boost (although I believe revo is pretty low to start with) and try and keep it on boost for longer, but IMO will make no real road difference.
When I had my exhaust done, did it make the car faster? Probably not, but it feels it. Im sure Phil will be happy when he does it, even if he comes back with it down on power. Just because it "feels" better, which is easy to convince yourself when you've just spent £250.
Im not saying it will or wont be better, all I can say is the difference will be so small that I doubt your could "feel" a massive difference.
I did consider this with Redcupratdi a while back. With uprated clutches we could up the boost and torque, but were told there is little to be gained once passed 200 BHP.
Edit- A diesel will never pull well at the red line. It just smokes
An im looking forward to hearing the Verdict on Friday
(If you have Vag com do some loggin Revo and Bell ) Would be interesting.