I've read all the debates about piggybacks here, theirs pros and cons, as far as the so-called respectable North American tuning companies are still promising an ECU tune for S4 B8 to be released "in two weeks" for almost a year by now. Sounds very promising.
I've made my choice - Cantronic for "430" horsepower has been installed into my car. It's become definitely faster. How faster? - you will ask. Don't know exactly - I will answer, because there ain't any dyno around my vicinity.
0-200 km/h for 16-16,3 seconds according to GPS and professional stopwatch. Going to measure 0-250 km/h time, but no such possibility yet.
But last year I raced my then bone stock S4 against tuned ///M3 e46 - ECU tune, intake (stock exhaust), camshafts, rev&speed limiter removed (9000 rpm), lower suspension, short-shifter, carbon fiber hood and trunk lid.
Roll-on 40-200 km/h from the 2nd gear: dead even until 180 km/h, where tuned ///M3 slowly began to take off. Not a chance for S4 at high speeds.
Now, this year, with my "looks-like 430 horsepower" Cantronic-piggybacked S4.
Same cars, same road, same drivers, same distance. But now not a chance for the ///M3. From the very beginning of the race (30 km/h-200 km/h) S4 takes off for about 3-4 car lengths. At the higher speeds the distance between our cars increased slower, but it definitely increased.
To my and ///M3's owner (very experienced fellow, his Evo VIII ran 11,8), the amount of horsepower's increase in my S4 looks like 60-70, which, IMO, looks closer to reality.
Just my 2 cents. Cantronic IS expensive, but it works.
No other mods in my car, just Cantronic alone.