Was a lambda test done, since it has a cat, or was the cat test not performed because the car was registered before August '92? If the lambda test showed low lambda, you have a rich mixture and can suspect the air mass meter, lambda probe or fuel pressure regulator.
However, when my Aero failed last year for high lambda (weak mixture, which turned out to be due to a Trionic sensor), the CO was high but within limits. One of the things I tried for the re-test was injector cleaner. This didn't make any difference to the lambda (it just touched the limit briefly so passed - I fixed the sensor later) but the CO went down to almost zero.
Worth a try, as dirty injectors will give a poor spray pattern which will affect running and emissions at idle (the engine doesn't seem to give a **** about injector spray pattern under high load - "just gimme loadsa fuel any way it comes").
Is your car an '87 or '89, or do you have one of each? (You mention an '87 SE in your post about boost pressure).
And welcome to SaabScene, of course.