I had issues with a Gen 2 model 17 that would bop me in the face and head with ejected cases. I bought the gun used and have no idea how many rounds it had through it.
I had two master Glock armorers take a look at it and try to file the ejector to no avail.
I sent it to Glock, and got it back with a new problem that I had never encountered in the gun--horizontal stovepipes where the ejected case would get caught horizonally in the ejection port and interfere with the next round being fed.
The gun came back from Glock much worse than when I sent it there.
I then had a Lone Wolf extractor installed and an HRED and I was back to getting bopped in the head but no more malfunctions.
So I sent it to Randy Lee of Apex Tactical and he lowered the ejection port and did some tuning with the extractor and now it works fantastic.
Below is a picture of a normal Glock 17 on top of the one that Randy worked on. You can see the lowered ejection port on the bottom one.
I don't know that this will help anyone with Gen 3/4 problem Glocks, but Randy definitely addressed the issue of me getting bopped in the face and head with ejected cases.