I never though of that to check distance using "pick by comparision" but i must admit, that's the best one so far.
I start with 10 "A"'s on screen, after destroing one "A" it will spawn 10 more..
...i got bored after having over 2500 "A"'s on screen :)
keeps framerate at 60 - start's to be a bit jumpy (56-60fps) after having 1000 on screen and you spawn some more, but it's only for "spawn time", then it back to 60fps.