I've shot quite a lot of the subsonic .22LR. Mostly Federal (900 A & B), Lapua and several types of Eley mostly out of a 10/22 bull barrel with lots of aftermarket parts I've added to it.
Accuracy: excellent for all. Obviously, some more excellent than others. Some guns like one type, some like another.
Will it kill the critters?: Absolutely, they won't be able to tell the few fps difference, at least not for long.
Out of a Mk II pistol, it is obviously a little slower, but the two answers still apply.
Now, if your talking sub-subsonic, like the Colibri round, then the answers are 'no' and 'probably', that is if you can hit them just right. But it is still fun to shoot the stuff, it is sooo quiet. Out of a 10/22 or a Marlin 39, all you hear is the hammer and the plunk when the bullet hits the target. Out of a Mk II pistol, it is fairly loud sort of like a high power air rifle.