I bought my Glock 21 in 1991. I cracked the inside corners off the frame some years ago, but I continue to shoot it with no problems. The pistol has between 13,000 and 14,000 rounds through it now. Most of them in the early years when I had hot-rod fever.
I've fired the Lee 255 RF at 982 fps and the Magsafe 66 grain Swat loads at over 2100 fps. I unsure which load cracked the frame. I just took a knife and trimmed the corners off since they were battered back and cracked. Hasn't seemed to make any difference. Most times I run the origional factory captured recoil spring, but sometimes I substitute a Wolff 24 pound spring.
Several weeks ago I fired some Lee 252 SWC's at 857 fps.
The pistol used to love the Lee 200 grain SWC. Gave great accuracy and function. I had some failures to eject with the 252's. I think it was just me limp-wristing the pistol as it stopped once I took a more aggressive grip.
I have even run some paper patched cast bullets through it at various speeds.
Keep your Glock digesting standard pressure loads and it will do just fine. Mine still does well, though it has been abused. Most of my loads have been cast lead. Yeah I know the old wives tale, but I don't buy it.
The day I brought mine home I thinned the grip with a hunting knife. Last year I shortened the grip and cut the swell off the back of the grip. I filled it in with black Acra-glas, reshaped it and then textured the grip. It's now a 21/30 and I prefer the shortened grip.
I'm betting it will still be going strong, even with it's cracked frame, long after I'm gone. Flexing is the secret to it's longivity. Wish I was still flexible. Ugh.............Mike