3hp? You mean 3-5 times the normal variation of back-to-back dyno pulls?
I'm not out to bust your bubble, but you can't characterize or prove a 3hp gain. And if you could, it wouldn't make much difference.
(and I run a 10% UD WP pulley myself)
Edit (now that I'm sobered up a bit and have some coffee):
The 3hp number has always been a pet peeve of mine for the above reasons. While it probably saves a bit of power, I think it'* impossible to determine exactly how much. There is a RUMOR that our water pumps cavitate at the high end, and run more efficiently with an UD pulley. I personally think my 10% is too much for a daily driver. Not sure what the ZZP is.
The bad side of this for the S1 L67'* is this will prevent using the 3.5" idler if you have a smaller SC pulley. The idler will hit the WP pulley, and the belt will rub against itself. This is part of the reason I added another idler off the ICM bracket on the Zilla.
But at lower RPM'*, the cooling is less efficient (at least on my car). With the OEM and replacement water pumps. This is part of the reason I run a pretty heavily drilled 160 thermostat now.