Keep in mind that it's not a slide release....it's a slide stop. You can look it up in an armorer's manual. I slingshot the slide anyway, regardless of the gun. Much easier under heavy stress, and although it's a little slower, with practice you can get quick.
If the grip is too slippery when you shoot, use skate tape/decal grips or AGrip....a lot of people like the latter.
As for the mags, the AWB is about to sunset, so you should be able to get some higher capacity magazines, no?
As for the tennifer finish, I have never had a problem with mine.....I've done one handed clearance drills using the slide of the weapon. I had tons of paint marks from the object on which I racked the slide, but they easily wiped off with a silicone cloth, leaving the gun looking as good as new.
I think the 21 could be a bit thick for some to carry, but if you really dig this weapon, you'll find a way to carry it.....or get the G30!!! It's *very* controllable, but some people don't like the grip length and backstrap.