It's really odd that a normally aspirated engine's MAP sensor will read boost. In GM land, one must move to a 2 or 3 bar sensor if running boost.

At least on the 97 C 280 / E 320 / SL 320, they use Bosch ME 2.0 / 2.1 engine control. I'm pretty sure the MAP is only used for OBD diagnosis and / or maybe transmission control but don't quote me on that.

Pre 97ish had a hydraulic trans so trans control would not be possible on early cars.

Given the sensor is Denso ( Japan ) there has to be a turbo car out there that uses a similar sensor that you can compare pressure to voltage.