We like to use a hub dyno because nothing such as tyre pressure, tyre grip, wheels size, wheel width, tyre temperature etc etc can effect the outcome. Its always the same, no ifs or butts.
If you use Temperature correction then the results are as real as they come. YES there is a difference between power at the hubs and power at the wheels. Personally i dont care if its making less than XX kW @ the wheels.
Personally i want the most accurate figure possible so the tuner can do his job even better. The torque accuracy of the hub dyno is second to none. It calculates diff ratio and will give you the actual torque created at the flywheel by the engine.
MPH times really aren't an accurate way of proving or dis proving a car/engine setup. Far too many variables. Different drivers, different cars, different suspension setups, different tyres, reaction times the list goes on.
Just because a car makes a gazillion kW on a dyno doesn't make it the be all and end all. But on the flip side a car that only produces a 'low' power output doesn't make it a car that can't perform and put serious numbers down.