I've never used one first-hand, so couldn't speak on any sort of actual RPM increase. I think they are in the 300-400 dollar range.
Check out youtube videos of Sound Performance Supra's, they have some where they compare with and without QSV dyno charts.
Flopster, I think you figured it out on your own
But if anyone else is still confused, yes you reduce the "A" by blocking off one of the scrolls. So if you're A/R was .90 (90/100), and you block off one port, you would have reduced the A (90) to a lower number say 50. So your A/R is now .50 (50/100). I am not a mechanical/math master, so I don't know if it's exactly cut in half or what. But it is significantly reduced.