We always knew turbo chargers turbines  had excess power over
and above the requirements of the compressor. That is why the Mazda
stock 2nd RX7 turbo's we were using were blowing up at 12K altitude after
only a few hours. I think Mistral blew two or three. John Slade maybe two.
Dave Leonard at least one. Apparently they were over speeding to well
over 100,000 RPM. This combined with the hot exhaust gas temperature caused
blade failure.

We knew they had excess power because the turbine was able to spin
up the compressor rapidly in fractions of a second for acceleration
purposes in the RX7.

I just did not have any numbers on it until now.
This pdf paper puts the simple numbers on turbo compounding.