Seen on another thread someone who had their S5 carbon cleaned
Whilst strapped on a dyno, with a before and after run
Without being moved the whole time
There was an increase of around 20bhp with a print out
The old n'asp V8 S5, or the more recent turbo one?
You will defo get gains on n'asp engines as they ingest a fixed amount of air. And it's got 8 sets of valves clogging up, which when summed together is a lot more of a restriction compared to 4 sets of valves on a 4 pot turbo engine.
It's getting on for a £400 job on the TFSI, which is too much money for such a tiny gain.
If someone did some MAF logging at idle, part throttle and WOT and found an increase in air flow being pulled in under all those conditions, I'd probably do it, but if the engine runs smoothly and is quick, I'd leave it personally.