I used "GM top engine cleaner" (same or similar to X66?) on my 300E, and if the billowing smoke after use was any indication of the product loosening and burning off carbon, it worked very well.
However, a few thousand miles later I pulled my head for a valve job and found a good amount of loose carbon flakes in every cylinder.
I suggest using the X66, or GM top engine cleaner ($6 from any GM parts dept) and afterward, vacuuming out the remaining carbon from each cylinder bore thru the spark plug hole.
If used properly (follow instructions on can to a T), these "top end" cleaner products work very effectively.
'91 300E, 213K miles