I would be interested to find out how many people have had there car chipped/remapped then have attended a RR somewhere else and what the bhp difference was.

Why ?

Do you mean - are there differences between rolling roads ? Yes, definitely.

Or - Is another rolling road going to show less power than the place where you had it modified? Maybe.

To be honest though, I've given up bothering as with Turbo engines, the atmospheric conditions on the day can have just as much impact as the difference in tolerances between two RR's.

The best measure is to stick with a reputable tuner (ie One that no-one has a bad word about) and then get a before and after comparison. The % gain in power and torque is what is important, not the absolute figures which can become pretty meaningless.

As an example, I've a 4x4 also, after several rechipped/remapped Subarus and Audis. On the day my Skoda was remapped it was scorching hot and I specifically asked for less outright power and more in the mid-range. Still ended up with a gain from 164 bhp to 218bhp - about a third more ? That'll do me, irrespective of the accuracy of the 164 and 218.