Lenny,
IMO, I would reconsider exactly just what it is you expect out of this rifle.
I would not consider H4831sc or H1000 ideal powders for a 'gas' gun.
Look at your posted data, accuracy node at 2870fps with H4831 and same with even more powder with H1000, same speed and no more capacity with H1k.
I would consider H4831sc the slowest powder I would use in a gasser.
With H4350 you said speed ~3000fps, but accuracy was so so, did you try going slower with H4350? I would bet there is an accurate lower velocity that is faster than the other 2 powders.
Just something to consider.
On a side note, I know a fellow competitor who shoots a 20in gasser in .260 remington and H4831sc. In competition, on his first string of ~20 rounds, rifle shoots fine. 2nd string he starts experiencing
FTE, and FTF's. By the 3rd string he is tearing his hair out clearing jams, and this is in a single feed format. I believe this is happening because his gas system has become clogged with the slower burning powder.
Again, my opinion. I would be interested to see your continued results.