I recently installed a UD underdrive pulley and was very surprised at the difference it made.
I'm not talking butt dyno, since that lies. But the big difference was I have had to adjusted my off the line behavior because it is that much more responsive without all those parasitic losses. It does greatly improve throttle response.
And if you are going to get your clutch done any time soon, lighten your fly-wheel. I haven't done this on my Subbie, but had on my Honda, and it makes the same sort of difference. Just not cost effective unless you are already doing the clutch.