Good post, and good shooting!
I've long been a fan of longer distance shooting, though as many have found, you get a lot of people that simply dont believe you if you talk about it. We have an 18" plate @ 300 yards, I've probably shot at it more with handguns in the last 6-8 years than any closer distance, though I've been shooting distance for 30 years or so. The 24" 600 yard plate is a bit more difficult, I've only tried it once, but managed a couple hits out of 10 rounds fired with a g-19. That takes a spotter. Once the distance starts opening up, the velocity fo the round being used starts to make a big difference. Making hits at 300 yards with a 45 auto takes waaay more sight than a 9mm, or anything with a similar velocity spreads. 357's are good at this game.
Regarding the eyepiece/hole thing, Merit has long made a device that does this, and is adjustable. You dial it until your image becomes sharp. You can suddenly see both sights and the target clearly. I have one, but usually forget to take it. They work like magic for aging (or not) eyes. This is one thing that does exctly what it says, I'm surprised more dont know about them or use them. They also make an adjustable aperture for peep sights that is supposed to do the same thing. I haven't tried one. The guys that use tang sights on lever guns say they sharpen up the vison when the correct sized aperture is used.