If the filters check out...

Check that all the vacuum hoses behind the right headlight are connected as they should be. The ECU uses vacuum actuators to enable boost. If there's no vacuum to close the wastegate, default is full open and effectively no turbo.

IIRC there's no way to replace the alternator from below. You have to remove the turbo air intake hose and pull the alternator through the gap. Very easy to hit the vacuum hoses behind the headlight in the process.

Sixto
87 300D