just got a reply from trey:
Yes, the numbers we input for vehicle weight help the dyno properly load the vehicle. It's a load-based dyno, not purely a inertia dyno. It simulates inertia. It also uses an aerodynamic profile for loading.
Ultimately, it's impossible to compare numbers from one dyno to the next. Also, Mustang dynos have the ability to compute numbers based on parasitic drivetrain losses, and other correction factors. Some shops run their dyno to estimate flywheel HP, while others use different methods for coming up with their final numbers.
Ultimately, all you can do is compare the numbers the car made stock to what you make after each modification.