I just used 2 cans of the Subaru upper engine cleaner on my MPS6.

I ran the first can with the engine off through the brake booster hose, then had a brainwave and ran the hose for the second can underneath the silicone hose just before the throttle plate at idle.

It only smoked for a little bit when I started the car afterwards, but coupled with an oil change it's now much smoother at idle than before.

I'd recommend it