man, just push the I believe button...
Alright, I'm going to do my best to try to explain this. Hopefully it comes out like I mean it too. I'm not at my best at 4 in the morning.
Yes, in physics equations, a fast moving light bullet has more energy than a slower moving heavy bullet, but...
Out of the short barrel of a pocket gun like you mentioned, you are not going to get the full benifit of a hyper-velocity ammo. Usually, to get the kind of speeds they brag about, you are going to need to use a 16" barrel, not the 3 inch barrel of a mouse gun.
The subsonic round is likewise going to suffer velocity wise, making it even more subsonic, but it is going to reach a higher percentage of it's design speed, due to quicker burning powder.
Finally, by using a slower moving bullet, you are less likely to have over penetration. Rather than a 30gr bullet speeding on through, the 60gr subsonic will deposit all it's energy in it's target, in effect, 'hitting harder'.
Phew. For more data, go to
www.naaminis.com and read their message boards. It could be I'm all messed up....there has to be a first time for everything.