I vote for this.

My STEn-converted M11/9 with stock upper ran so fast it sometimes would get ahead of the 70-year-old springs in some of the STEn mags. So IMHO, a mag which fails to feed consistently at 1,000 rpm may be 100% reliable at 650 rpm.

FWIW, there was enough manufacturing variation when STEn mags were made that some may fit your gun fine, others may not. So the first thing to do is to test-fit every mag in your receiver. Put aside those which sit too high or too low, too tight or too loose.

Of the ones that fit fine, strip 'em down and clean them (some have never seen cleaned since they were made in the 1940s). I just run 'em through the dishwasher on "pots/pans" cycle when my wife isn't home. Then lube and reassemble them. In the process, you will note some variation in spring strength –– set aside any that appear on the weak side.

This will prolly cull out 10-15% of the mags, and you can always tinker with them later. The remaining mags will have an overall higher chance of running reliably than the rate for the entire batch.

HTH.