I had heard trombone players speak of this, their upper lip does enter the cup a bit and touches the cup when they vibrate. How about horn?
I had heard students say they felt the lip hitting the cup, and then recently myself I finally noted this with the Houser San Francisco cup and the Houghton H-4. They are both fine mouthpieces that I like a lot, but as I use a plastic rim I could feel my lip touch the cold metal of the cup. I suspect it relates to the “weight” of my lips (somewhat heavy) compared to the top of the cup, which tapers quickly into a smaller diameter cup. Many players I am guessing don’t have this same issue with these cups.
I’m not sure if it is something you can hear or not. I’m going to say that it might be slightly audible at high volumes. In my own personal case, if only due to my metal allergies and my lip touching the cup, over break I made a decision to switch off both of these cups in favor of the Houghton H-1 and H-2. I like both of these a lot too, and both are a made in such a way that they don’t have that quick taper at the top of the cup, at the joint between the rim and the cup. My lips do’t touch these cups.
As with most things, your mileage will vary. But it is something to think about, it will impact how you or your students feel about a mouthpiece.