Cor-Bon makes a 150 grainer for the 7.62x39 that scoots at 2300 fps or so. It's quite accurate compared to most steel cased stuff. Wolf makes 154 grain at 2200 fps and it's more accurate than most 123 grain loads. Not quite as accurate as Barnaul, but close enough.
When you consider the pointed (and often boat tailed) bullets of the 7.62x39 vs. the flat pointed bullets of the .30-30, they compare favorably, especially at the further end of their range.
I have never had much luck with hi cap detacheable mags. I have one SKS that I hand fitted some 30 round mags to, and they work "well enough" but I wouldn't want to trust my life to them. Maybe the Triple K mags are better.
I think the problem is a combination of loose tolerances in the SKS manufacture, combined with low quality of most detacheable mags. If you can hand fit them, and the mag itself will work, they shouldn't be too bad. I just prefer to use strippers on my other SKSes. It isn't worth the effort trying to make hi caps work.
PaulS, what cartridge did you convert yours to? That sounds like an interesting setup. I'd like to tinker more with mine and something like that sounds good. Thanks for any info.