BMW and Porsche are really the only manufacturers that rate by wheel HP.

No they don't. This is an internet myth repeated by people who don't actually know what's going on.

There is a standard procedure for measuring certified horsepower figures. This standard is called SAE J1349. It has a few requirements, but the relevant one here is that it requires the engines be run continuously at peak power until HP stops falling and stabilizes (usually 5-10 minutes).

On N/A cars, this usually doesn't make a difference in power. But BMW and Porsche use the regular cooling system to cool the intake charge for their turbo motors. They will overboost the engines until Intake Air Temperatures hit a threshold (or a time limit), then the DMEs will switch over to a low boost table. This low boost table is what the HP rating is measuring.

Dyno operators don't run customer cars for 5 minutes at redline. They just do a simple 30 second pull. They run "SAE correction" which is simply correcting for air temperature and pressure. It is not making the HP rating SAE J1349 certified.

If you dyno a BMW using the certified procedure, the output will be the certified HP figures. It will be bang on too, since modern BMW engines are programmed to request specific torque output. Run a B58 long enough at WOT at 5,500 RPMs and the DME will request 500Nm of torque instead of the 550Nm requested in the high boost table.