I was under the impression that when you get a custom remap you can specify bhp/torque whatever....depending on what you've done under the bonet etc...already. I could be talking complete crap though!
Not quite. You don't pay for a custom map and say 'I want 300bhp and 300lbs/ft torque please'. You leave it entirely to the tuner doing the mapping and they will sit and go through each parameter to work out how far they can safely push things.
With a standard 'one-size-fits-all' map the same procedure is done on a standard dummy car and that is then passed on to all other cars regardless of the state of tune.
With a custom one they will probably start off with a standard map and then start tinkering to see if anything further can be adjusted. For instance, if you have an exhaust, CAI and FMIC the tuner may be able to up fuelling and boost knowing that the car is breathing easier and charge temps' are being kept down more than a standard car.
A custom map doesn't guarantee more power and torque, it just means it's individual to your car, that's all.