rolling roads have hundreds of uses from a simple diagnostic tool to test for missfires etc... or an essential tuning tool for professional ECU calibration or remapping.
Some read lower figures than others, some read higher, they all use different calibration methods but if you find a good rolling road tuning centre you wont go far wrong if you use the same centre to test before and after modifications to see the power gain (or loss)
A tuner that has invested in the right tools i.e a professional dyno will probably have invested in real training and be proficient and passionate about what they do, after all they have spent tens if not hundreds of thousands of pounds on premises, dyno, staff etc.... you dont get this from a mobile tuner with a van and a laptop so choose your tuner carefully.
The number I posted is for Mike at DD (dyno dynamics) if you get your car tested on one DD then it wont be much different on any other DD in the country as they are all calibrated to the exact same tolerances and use the same calibration methods and systems.