Haven't really heard of any real increases in HP. Since we use 2 stroke engines, the exhaust and intake ports are open at the same time, so all the air coming from the supercharger just ends up going out the exhaust port and you really don't see a big HP gain...if any.
Now if they were to make one for a 4 stroke...that'd be a different matter. And there are supercharge 4 stroke engines...YS engines are supercharged, but they're for airplanes. I'm sure they could easily adapt the supercharger for 4 stroke car engines.