i don't have any of the c-products 5.45x39 mags to test out, but since there not that expensive i'll likely order a couple one of these days.
with my limited testing on GI,D&H and magpull p-mags with the 5.45 round this is what i discovered.
the mag-pull p mags were the worst--they wouldn't feed more than about 7 rounds. any more and the 5.45 would sometimes stick down inside the mags.
the D&H steel mags fed up to 24+ rounds fine--but when emptying the rounds with the mag out of the weapon the rounds looked to be only held in place by the very back of the feed lips. when taking the rounds out of the mag by hand they were tilting around all over the place. to looking at this it is hard to see how it fed the rounds while in the weapon. these were the mags i used during my testing the other day. the spring would also get soft at about the mid way point. they seemed to work but i certainly can't trust them for 5.45 rounds. the D&H steel mags were the same with the green GI followers or the anti-tilt types.
so i dug deep into my old-old duffle bag and found some very old USGI aluminum mags and some really crappy made in china gun show junk mags that i bought back during the clinton AW ban.
these mags with the green followers feed the 5.45 about perfect. ironically the china junk mags in praticular love the 5.45.
after loading the rounds in them up to 28-29 rounds. they strip out easy, no binds, no tilting, no soft springy-ness anywhere in the mags while loaded up. i don't know that real 5.45 mags could be much better than these make-dos.
the moral of the story is. if you go with a 5.45 upper you may or may not need new mags. the new-hi tec mall ninja mags may not be the way to go with the 5.45x39. if you have some old mags that were questionable with 5.56--check them out as they may run 5.45 very well. from what i have seen so far the super tight fitting mags are not the way to go with the 5.45. the least expensive aluminum mags you can get your hands on may the best for 5.45