Cant speak for the 20 inch barrel. But i found 45 gns of 2208 worked well in one of my Omarks with 155gn SMK, 28 inch barrel though. I think if you load too much powder, it wont get all burnt before the bullet exits the muzzle, so maybe not ideal. Maybe you will find the node, on the lower side of the powder listed loads. Keep us posted on your progress!
Not had to do any 20 inch barrel stuff. But my initital thoughts would be to try faster powders in the mix like 2206H and even 8208. I was surprised to learn about 8208 for the 308 being used very successfully in F class not very common but the guy using it was on top of the podium so it was producing results.