Just got back from the range. After 125+ rounds, the stovepipe problem is not as bad as before - ~2-3 FTFs for each magazine load. I noted that they usually occurred as the last round was up for feed, and about halfway through the magazine. So there you have it. These "CZ mfg" mags are definitely Khyber Pass quality, but CAN be used (prior to the mod every other round was an FTF).
So there is some hope if you can tolerate some annoyance. Hope this helps. Me, I'm on the lookout for real surplus CZ magazines...