Regarding the Glock and heavy hard cast loads, my experiences mostly mirror GJMs. I've been shooting Glock 20s off and on since the early 2000s and carrying them as field pistols in the Pacific Northwest. I've owned Gen 2, 3, and 4.
The Gen 4 seems to have a more robust stock recoil spring, but I still encounter problems with heavy hard cast loads. I have a 200-grain XTP load at 1140FPS that runs quite reliably both with a strong, two-handed grip and WHO, however subjectively it feels like it's on the verge of choking when run WHO only with a loose grip.
The heavy 200 grain, wide meplat loads are a real challenge for this gun. They weren't designed for it, and the bullet profile isn't a good fit for the magazine. I find that they will usually run ok with a strong two-handed grip, but will choke fired with a WHO loose grip. I feel that evaluating reliability in this fashion is a valid test, as people often find themselves shooting with a less than optimal grip due to injuries, or a bear chewing on them.
I suspect that with the Glock 20 you can have any two of the following three things: 1) A 200-grain bullet 2) 1200 FPS velocity or 3) A wide meplat bullet.
I'm pretty happy with my 200-grain XTP load for where I live where the threats are black bear, cougar and meth-heads. One of my future reloading projects is to develop a hard cast load that will work with the stock factory barrel and recoil spring. I suspect by dropping the velocity and maybe the OAL, one could make a 180-200 grain hardcast load in the 1050 to 1100 FPS range that would work well. It just hasn't been a priority for me.