Anybody had there car terracleaned ? If so did it make much of a difference ?

Nope sorry. I know my father in law paid £180 to have his done at a body shop once and it did come back pretty damn nice if I'm being truthful 

Just had a look round some of the other forums, this seems to be typical of the effects, sounds like it does the business:

 

http://www.bmwforums.info/bmw-chat/8771-terraclean-treatment-review.html

Not heard of it before but certainly deserves more investigation.

It would worry me having some guy unfamiliar with Subarus messing around with something as important as my engine.

had the terra clean done. improved idle, smoother abd better throttle response, not sure on if it improved economy yet. but the best part ... car past emmisions test on mot even withouta cat !!

nice one it's definitely something to consider if only for emissions then ;)