I have not done this nor had my dealer recommend it. It really depends on what they're cleaning carbon build up off of/ out of. Frankly, should be covered under the extended emissions warranty but different topic.
If they're suggesting the diffuser, I'd say do it yourself:
Same thing with the MAF sensor on the back of the engine. pretty easy to get to and gets clogged up with carbon as well
If they're suggesting the diffuser, I'd say do it yourself:
