Had mine done by Duncan at Surrey rolling road too. Nothing but praise for the bloke. I'm sure he's always busy and bombarded with the same questions but he had plenty of time to answer me and he even visited me at home for a little tweak.
Main thing to consider between dyno and road mapping is the car/engine will be under different loads. Air resistance, road resistance, inclines will all have different effects compared to a dynos rollers. It'll still be mapped safely because he can monitor detonation and boost, you just won't get a dyno printout with a power figure.