There's a bunch of posts on various forums about the extended mag release that HK Parts (among others) sells for USP pistols (which is actually just a HK45c mag release) doesn't work reliably in the 40 and will sometimes release the magazine during firing.

All of these posts seem to be years old however so I was wondering if this is still an issue.

For some people they work fine and others they don't, the problem is the extended levers have a metal insert in them where they contact the mag body, to lock the mag in place. The USP compacts and the USP 45 full size have metal mags, so it is a metal on metal contact and the extended levers work great with these.

They don't work for some folks when used with the USP 9 or 40 full size because of the plastic mag bodies and there is the potential to chew up the plastic on the mags as well.

Interesting.

What about using the (plastic coated) metal jet funnel mags, any idea if that makes the extended mag release more reliable?

The only issues I’ve ever had with running the 45c mag release in a USP is that most kydex USP holsters are molded for the factory mag release and can push on the extended one, unlocking your mag when you move around, sit, etc. After reshaping the holster it was ok, but I also didn’t keep it on the gun long for other reasons, so at least on mine I didn’t really encounter any issues with it dropping mags at the range. Not after making sure the mag was seated in the first place before shooting.

Personally I’d be leery of the metal mag release chipping away at the plastic mags, but since we’re no longer in a preban/postban era, you can always replace a mag if it gets messed up. Obviously, YMMV