If the effective gearing changes that are due to the tire diameter changes the horsepower that a dyno reads, then the dyno is not very good at reading horsepower. Actually dynos do not measure horsepower anyway - hp is just a calculated number based on torque and rpm. The dyno reads torque and rpm and then calculates hp from those two inputs. There should be a place to enter the tire diameter in the dyno's computer so that the difference in effective gearing does not change the hp, because changes in gearing just do not change horsepower (except in more extreme cases where the frictional losses change).
Also as far as rotating mass is concerned, if the car accelerates slow enough while it's on the dyno, the mass becomes a negligible factor. The faster the car accelerates on the dyno the more the mass will make a difference in the WHP. If a load-based dyno holds the car at a certain RPM and reads torque then the mass makes no difference.
Because of these and other factors, such as run-to-run variability in dynos and automobiles in general, it is silly to speculate about small differences in two diffent cars' HP as measured on a dyno. The dyno is a tool for tuning and should be used with extreme caution during bench racing.
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