I did the purple wire thing on my '91 300SE. I had the normal 600 RPM idle in gear, but it had developed an annoying buzz in the engine I was just unable to isolate and fix, and my hot oil pressure was dropping to around .4 BAR also. Cutting the purple wire from the gear selector to the ECM, and grounding the ECM side makes the ECM think the car is in neutral when it's in gear, and raises the idle speed to about 700. The little low frequency buzz went away, and pressure went to 1.0 BAR. The only negative is it does tend to tug a bit more at the brakes when sitting at a light. I think my in-town mileage dropped a bit too, from sitting in traffic.
But, as stated earlier, your engine should idle at 600 in gear, so the IAC or the air flow sensor may need work.

DG