While I don't know the mods specifically you speak of, I was pondering this the other day shooting my Classic and noticing it was the most reasonable SKS I have ever had regarding finding your cases. It is also the most heavily fired SKS I have ever seen...much less owned.
The majority of the cases landed within an 18 inchish circle about five feet to the right, and a yard or so behind me. A couple shot straight up, and a few feet in front of me. I want to say they were the first and last. I only did a couple of strippers as this was a function test after rehabilitating the weapon to being safe to fire. Two different types of ammo both yielded similar launch and landing results.
It got me to contemplating the forces involved in cycling the weapon, and what might have affected it to do this. Unfortunately, I am leaning towards a lot of it being a number of factors, that might be hard to sync. Based on the sound of the gun functioning while firing, I kind of suspect that the recoil spring might be a little less forceful...which I suspect would influence timing. As far as the upward push of the next round happening simultaneous with the extraction, the follower spring force AND which side of the stack the next round is coming from would influence things as well. Additionally, the force the extractor puts on could come into play also, both with the spring strength and the shape of the mating surfaces between the shell and its grip.
I was forced to use a replacment stock and a borrowed gas tube, which while a little wobbly, I don't think affected pressure. A slightly longer handguard has since tightened it to normal, but I haven't fired to confirm any difference that may have made.
Not knowing about the holes in the gas tube you are talking about adding, it does make me wonder if you could expand the vent holes behind the piston head in the tube to reduce back pressure....but then you also could gain the same effect with a weak op rod spring.
All of this of course could also be adjusted by the size of the gas vent in the barrel. A variable gas valve might be the easiest way to fine tune for results....although I think the concert of spring activity involved might have more of an impact on results more than simply adjusting the pressure initiating the whole mess.
Now I know why I stopped trying to think about it, when I should have been happy coming home with 18 out of 20 fired shells.
Your circumstance sounds as if it's more mission critical for function, hopefully my pondering this lends some insight towards a solution.
Good to see you posting...it seems like it's been awhile.