Its hard to say exactly. Usually a hyper-velocity round like the Aquila Supermax or CCI Stinger would have a lot more power (I'm not sure if it would be double or triple the power but it would be in that range?), but that's in a rifle length barrel. In a pistol length barrel, the benefit from the increased charge of powder in a hyper-velocity round is probably minimal (since .22LR rounds take between 12 and 16 inches to fully burn all their gunpowder). In a 4 inch barrel, most of the powder charge won't be burned in the barrel and you'll not get the full benefit. (Probably all you'll get is a bigger muzzle flash as the powder ignites in the air in front of the muzzle.) Since that's likely the case, I'd recommend you simply get a 40 grain hollow-point loading. You could go with the CCI Velocitor (a 40 grain hyper-velocity hollowpoint), but I don't know that it would yield any great improvement in performance.
As far as using this as a defensive pistol, a .22 is better than nothing, but its far from an ideal choice. Because of its small bullets and low muzzle velocity, its just not as powerful as the military and police calibers (.38 Special, .357 magnum, 9mm, 10mm, .45 ACP, etc...) that are commonly used.