Seafoam is normally used to remove carbon deposits from cylinder heads, pistons, and valves. Why you think your turbo has a carbon problem escapes me. I guess you could pull a spark plug and examine the piston crown with a miniature camera.
Better than seafoam, is to run your car up to redline in second gear a few times. That will remove all the rough spots. Just watch out for cops!

There was a person on this board who tried to clean out his engine with water, same principle as seafoam and works just as well. He hooked a manifold vacuum line into a can/jar of about 8 ounces of water, with the engine running. Then he got behind the driver's seat and tried to get the engine up to 2000rpm to suck up the water. His engine died! Then he noticed that the water container was empty. He pulled out the #1 spark plug and all of the water was in there, keeping the cylinder from reaching top dead center.
Ouch!
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)