im here to say that you arnt gonne feel it on the ass dyno...1-3 hp...not gonna feel it. I have had my AFX pullies for about 3 years, and I cant really feel anything. I guess in the grand scheme of things you might feel it with a bunch of other mods...but by itself, you arnt gonna feel it. You will however get a little bit of headlight dimming when stopped somewhere, like at a stop light or something.
I've installed the AF/X UDP on several ZX2's and Zetec Focii at the track. The cars on averge dropped 0.2 seconds in their 1/4 mile time after the pulley was installed. There were on runs with the same 60' time on the same day at the same track in almsot identical weather conditions.
Stock UDP...hmmm. Like calling the belt tensioner a factory supercharger?
Be aware that the FC pulleys might create some belt squeak on startup. It only lasts for the initial cranking usually, but it can be a bit embarassing. I think it is the squeak of the belt against that oversized Water Pump pulley. Also, the pulleys are aluminum in most cases so the service life is going to be shorter than with the stock steel pulleys and they will create some aluminum deposits on your belt. A fine silver film visible on the unribbed portion of your serpentine. But the revving of the engine is faster, especially when teamed up with a racing/semi-racing flywheel.