I think I would ask kahr to send you a new slide stop. You should not be having issues with the slide stop doing it with all those rounds you mentioned.
have u tested to see if those rounds are indeed hitting that inside of the slide stop lever??? eliminate any possable chance of your thumb hitting the slide stop lever by accident.
they should all clear that slide stop lever...
I doubt if more rounds down range will solve that issue..
If kahr says they willsend you a new slidelock lever, then do alittle experimenting. Get a fine small file and take the lever off the gun and just file a tad bit and I mean a tad bit off the inside of the slide lock lever where it is hitting the rounds. You should be able to see that very easily by take the slide off and inserting the slide stop lever and then inserting a loaded magazine. You should be able to see those rounds hitting the lever, file just a tad bit in that area, smooth and try again, when they clear stop and go out and shoot it and see if it locks your slide open on the last round. If it does that then you have fixed the issue and just keepthe new slide lock lever as a spare. You arenot taking off hardly anything in that area, so go slowly. If the rounds brush by the slide lock lever, that is ok, just as long as they don't try to pick up the slide lock lever. that is what you are trying to achieve..
You can't hurt anything by doing this as a new one is in the mail and if you take off to much on the inside of the slide lock lever, it will indeed work perfect BUT it might not lock open on the last round.