Got a chance to head to the range with the Sp5K and some MKE translucent magazines today. First time using the Turkish made units.
I have to say that I'm impressed. I was concerned when test-fitting them, as they seem to have more fore-aft movement than either HK or POF metal magazines, which I was afraid might lead to feeding issues. But they ran 100% in my gun.
Only issues were with some leftover Winchester White Box and Remington UMC 115 grain FMJ. Both gave me a load of FTE's due to insufficient "oomf" to fully cycle the action. This happened from HK, POF, and MKE Plastic magazines.
I have heard that the MKE units may not run well in all clones (or even originals), but at >$30 each, it's probably worth testing them in your Mp5 pattern guns. Until Lancer or ETS make Mp5 magazines, these seem to be the cheapest non-ATI/Korean option.