[Unobtainium]

The 458whp was not exactly the stg3 kit, but similar parts.
That said, the average manual car shows a 15% loss, I'd expect the auto would show about 25hp more.
I really think the driveline loss should be expressed as a power number loss.
It should not change dramatically with bolt-ons.
If the car shows a loss of 65whp for example(manual car), then that number should be used for all subsequent tests on that drivetrain.
The % calculation is only good on the first test.
Generally, the autos show a further loss of 25whp, more on wild built autos I've seen.
Similar to how blower and turbo cars get false numbers on a dyno as the FI is not factored into the correction for density altitude.
A turbo car running even 7 lbs of boost cares a lot less about altitude than an n/a car,
never mind double digit boost numbers. It still gets the same correction factor though, spitting out a number that is higher.
I too would love a blower so that the car was properly equipped to run with GT500's,
But I'm not sure for the same reasons you mention.
N/a has always been my preference.