You need a load bearing chassis dyno to accurately read horsepwer and torque on a car with a stall convertor. Like you mentioned, with an inertia dyno they will show low horsepower numbers, but they will also show high torque numbers. If the inertia dyno is operated correctly they can still be used as a tuning tool, but comparing horsepower numbers is silly.
On a manual transmission car, its a different story. The Dynojet 248 I have access to is very repeatable. I have been on 3 different 248's and the corrected numbers are usually within 5-7 horsepower of each other. To me, thats close enough for me to quote what my car makes and feel confident that I'm not mis-representing.
Andy
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88 coupe
91 LX NMRA Pure Street 5120
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