I just read the specs on the 392. Looks like they're advertised about 684 fps second. That's about half of what a rimfire 22lr performs.
And as far as pumping it up a lot of times, rarely increases velocity because there is only so much internal volume in the onboard canister and there is a point of diminishing returns.
Continually pumping up the gun is much like over-filling your gas tank and won't make the car drive faster or give it more MPG either.
On the same note, if you fill the tank with cool air, (from inside in the AC) and then set the gun in the hot summer sun outside for a while, will increase velocity some because as the air heats, it expands. But even then, there is only a marginal velocity increase. Believe me, I've tried
You would get more velocity with lighter alloy pellets, and more thump with the heavier lead pellet, but will also decrease in velocity.
I would try the lighter pellets just to see how they do. You may be to get a little more penetration with them.