I have one fitted and the difference is very subtle, so really don't expect to suddenly have a racing car
you get a bit more pull without changing down a gear, and the reason I got it was I was fed up changing down to first gear trying to get up some steep swiss mountain roads, following my brother in his 59 vette!
The performance is very smooth (as is a healthy normal S4, tbh) with perhaps a bit more pull.
The one difference you feel is in higher revs, so overtaking on the motorway, or really pushing it on long twisting roads....but we are speaking above 4000/4500 rpm
Idling, starting, and generally poodling along thru town traffic is unchanged, which is also good.
I do not get worse mileage per gallon, which is good, as the super plus fuel (100+) here costs 18 eurocents a litre more! (basically 76 UScents per US gallon more)
The first chip they sent me didn't work!.... but they were very quick to send a replacement as when they checked the one I sent back it was indeed not correctly programmed.
Fitting was easy once they told me which way up it was supposed to go