I think we get way too involved in how much velocity we can squeeze out of a cartridge. That said, there is definitely a velocity floor, but it depends on your use of that particular load. Paco Kelly, among others, has written about the Keith 275 grain 45 caliber bullet at around 950 fps going through and through on a whitetail at 50 to 75 yards. Why do you need more? Dead is dead. Two bleeding holes are all you're going to get. The rest of that velocity just flings the bullet farther from its intended target.
Now, on self-defense loads, you want enough velocity to make the bullet expand. Well, how much is that for your particular cartridge? That depends on how the bullet company has designed it. Hornady xtp bullets seem to need more velocity, Speer Gold Dots less. You still need enough penetration to hit the vitals, and all of that is totally useless unless you place the shot where it can hit those vitals.
After all these years of experimenting, hunting, and pounding my hands and wrists with heavy revolver loads, here are the conclusions I've drawn:
45 ACP 230 can to as slow as 800 fps and get its job done.
44 Special 240/255 can do just about anything the world needs at 1000 fps and probably at 900.
44 Magnum 240/255 I'll load to 1200 just so I can tell the difference between it and the special when I shoot it.
45 Colt 255/275 will do anything necessary at 900 and probably at 800, but it's fun to make them go 1000.
357 Magnum will lose a whitetail doe for your daughter firing a 158 grain xtp from a carbine at 1150 fps at a mere 65 yards. With a 125 grain jhp at 1300 or better, it will light up a room, deafen you, and twist the revolver so violently, you'd better hit with the first shot.
9mm Parabellum 110jhp at 1200 to 1400 (HK-93) will scare the crap out of you when your best friend and two other officers pound a murder suspect who has just fired at them with a Ruger 44 mag carbine, hitting him 14 times in under 4 seconds. It'll take a center forehead shot to finally drop him. Your mind will understand that he still fell in 4 seconds, but you will never again feel confident with the 9.
38 special is a fine, all round revolver cartridge that's very accurate, and will do almost anything you need while hiking or backpacking. Since heavier bullets are available for it, you'll feel better with it than with a 9mm, then you'll carry a load that mimics the best of the 9's and feel okay with it. You'll understand intellectually that they are the same, but you just FEEL differently about them. :
So the floor here is the 158 LHP at about 700 or the 110 from Corbon at 1125.