I never had any on a mustang. But I did have an RKSport UDP on my cavalier. It was smaller and lighter then my stock one. After finding the right size belt for it, I noticed that the engine revved up and down faster. Didn't really notice any actual power gains. But I never dyno'd it and I'm terrible at sensing minor gains.
Steeda underdrive pullys are the only true underdrive. Any others are what they call piggy back and it can hurt your car in the long run. Steeda may cost more than any other but that's for a reason. I noticed a little hp gain from it but mainly the power I felt was in acceleration.
Bbk has a sfi crank pulley now but there more than steeds. I got steeds a couple weeks agoanf it revs faster. I thought they didn't do anything till like 4000rpms? My car barely sees 4000. So I think I wasted money