I took the armorers course earlier this year, and the only upgrades I like are the (-) connector and different sights. Anything else tries to make the gun into something it isn't and wasn't designed to be. That is of course, just my opinion.
With the Gen3 guns, I also did the Vickers mag release. But with the Gen4, the size and shape of the mag release works really well for me, and I can't see changing it, even if there was something available at this point.
I have DeFoor sights on my first Gen4 and they're great. I was going to get another set, but heard really good things about the design of the Warrens from some very knowledgeable people, so figured I'd try them out.
Ive got a new G21 SF that's gonna need a sight upgrade as well, but I think I'm gonna go back to Ameriglo and try the Hack sights on it.