I think they must do something, but how good im not sure. Doing it on a DI engine it would only really clean the exhaust valves and ports. For the full effect youd need to go to someone that used the one where it feeds into the intake and for that you can buy stuff your self and give it a bash. Go look up Project Farm on Youtube as he does a test to an old ford pick up im sure and the difference is mega! Think it was all seafoam products.

Id honestly give water meth a go as well if its not to late.
When i mapped my 335d, I didnt delete the EGR to being with but ran a WMI set up. I owned the car from 40k -115k and then i did the swirl flaps at 112k the intakes were spotless! Honestly put that down to running the meth set up. How long it takes to work im not sure, but if i had direct injection engine that was notorious for carbon build up id get it done.