Any experience with these? My M16A1 is still awaiting ATF form 4 approval, and I checked it out recently and the factory trigger was surprisingly good. I’m sure a Geissele SSF could only make it better, but I also have a HK MR556 that I’d love to shoot full auto one day. A Geissele HK416 trigger is the only way, right? Would I still be able to safely use regular DI uppers without risk of damage to the lower or upper?

You can use the standard M16A1 FCG you'll already have (for both), but yes,  The Geissele 416 will be better, and also works with any upper.