IMO, the only advantage of the 2.7tt is the turbo noises. But the additional work required to install the engine that is attributed to the turbochargers of the 2.7tt seems like almost the same amount of work as would be require to simply turbocharge the 4.2L... so... there's that to consider...
Oh wow I didn't know they could do that with the lights. Definitely good to know!
The nice thing about the motronic ecu into motronic body was that the CAN bus was compatible. My car was actually emission tested with the 2.7tt in it and passed without issue. As far as pin-outs with your car goes... I'd imagine that Porsche follows the same protocols that VW/Audi/BMW does in this link, considering they are all Motronic based.
https://en.wikipedia.org/wiki/On-board_diagnostics#OBD-II_diagnostic_connector
It would be interesting to see what happens if you wired the GM CAN wires into the Porsche CAN circuit. I'm not an automotive controls engineer... I just work in the plants and figure out how to assemble them and keep the lines running... so you'd probably be better versed to figure that one out. Are the communication protocols consistent across all makes, whether it be a Motronic or Delphi based ECU...