Have you measured the stems on the slider samples with calipers to see how closely they match the expected dimensions? In light of the recent issues with Kailh Box switches, that's probably worth checking. Is there a measured drawing showing the specified dimensions and tolerances for the sliders?
The manufacturer measures them and sends me the info, they use much more accurate measurement equipment than calipers(which aren't that accurate in most use cases), so I know what the dimensions are per cavity in the mold.
And I have to supply a drawing including the tolerances for the manufacturer anyways, but don't want to share it publicly. All of them are either 1.28mm or 1.29mm and that decision was made long before the issue with the box switches came to light, I was trying to undershoot the BOX spec because I didn't want the stem to be that large and stretch caps, thankfully I went lower because of what transpired.
I will check GMK soon as I only have PBT MX caps atm.
EDIT:Just tested with a GMK blank I had it a bag.
Does it stretch the cap? Sadly, yes it does, there are visible stress marks on the cap.
Does it make them loose on cherry mx switches? No, the cap is still tight on a mx red switch even after putting the cap on the nexus slider ~30 times. It is not nearly as tight, but it doesn't fall off or move on its own accord and still takes a bit of effort to remove.
It does suck but since the whole box switch issue happened along side this, it was impossible to predict or correct for. I decided 1.3mm +- 0.02mm way before the post for cracking was made, and even after the "fixed" switches showed up the mold was already machined or being machined and there was really no way to change.
Should you be worried? Honestly I don't think so, as even a normal cherry mx red fits snuggly still. In the future I will make an effort to change this, but seeing as I already had to reach out to get the money to make the last changes, there is no money left to do so now as I make nothing off of this GB.
TL;DR: Yes it stretched, but smaller stems still fit well, not enough stretch to worry.