Some time back I read an article in a gun magazine about this, where a firearms instructor at the FBI training academy said, "if you get into a face to face gunfight, you can be sure of two things: one, you'll shoot one handed, and two, you won't use the sights."
That's probably true. If you're pulling and snap-shooting, it's probably a situation where your attacker ALREADY has his gun out, and the extra second it takes to see your sights could easily get you killed.
You should certainly practice enough with your gun to be able to whip it out and hit an 8 inch paper plate at 15 feet. A challenge here is that a lot of popular self defense handguns, especially those carried concealed, have TINY little barrels, many under 2 inches. Even WITH sights, it's hard to hit the freakin' side of a barn with those things without extensive, regular practice, not to mention under the pressure of a split-second self defense situation.
I can hit that paper plate all day with my Ruger Blackhawk, firing instantly from the waist, but then it has a 6 1/2" barrel and I've shot a gazillion rounds through it (I reload). If I got a little concealed carry gun like a S&W Bodyguard, I'd have to go back to Square One.
Just my $.02 worth.