F=M x A. That's all you need to know, IMO.
If I want to compare two types of ammo, I just multiply the grains by the FPS. I don't know what type of units it results in, but the higher the number, the better. YMMV.
I've read over at Stephen A. Camp's site, Hi-Powers and Handguns, that if you have a 147 grain 9mm round out of a short barrel, you've basically got a .38 Special when you compare velocity and energy.
Seems to me that if you go below 90 grains in a 9mm with a short barrel, you've got yourself a nice .380, maybe even a +P, but that's just my opinion. As for me, I'll stick with whatever flavor 115 gr JHP round that works in the pup with a good track record, not the latest "gimmick" bullet..
For me, my ideal load for the pup would be the Federal 9BP JHP (not the hydrashock) bullet at around 100-105 grains with a nickel case. Could this give the best performance and reliability for the R9 ? I would like to build it and find out, if I had the resources and time (And Federal does not market their bullets as components anyway). I arrived at this conclusion based upon what I've read from (ok, flame suit on) the Marshall and Sanow one shot stop numbers for the 9BP load. But I've just created a new load here on paper, so all of that data is probably not applicable in this instance.