I understand that the head of the piston lies behind the nut. It creates the seal. The portion passing through the nut is essentially a pushrod. It's been mutilated. I can't help but think it could be allowing the piston to tilt slightly, especially if the head is a loose fit in the non-GI barrel.
There's the whole thing about the missing mass too. That would cause the piston to react quicker but I'm not sure if it would have an appreciable impact of the force applied. Lighter bullets moving fast are easier to stop than heavy bullets moving slow. This whole train of thought may be getting too deep and may not make any difference in the operation of the rifle. Theories are wonderful but facts make the world go 'round. The new parts will be in next week so I was just playing with what I had to work with.

I keep reading the signs as too little movement on the op rod. There doesn't seem to be proper force pushing the slide rearward.
Closing the vent would put a little more push behind the piston. Considering the angle of the vent it looks like it would be exposed when the back of the piston passes it. This is pure speculation since I can only see threads on the nut behind it.
I did not weld it closed. That would mean being dangerously close to welding the threads of the nut to the cylinder. I closed most of the exit without filling the void underneath. That will not increase pressures dramatically. At most, it will increase the velocity of the gas exiting the port. Pressure behind it will increase slightly but is shouldn't be an appreciable amount. It's more of a split second timing kind of thing.

Cannons, I didn't close the hole in the barrel. I closed the vent that allows the gas to escape the cylinder after pushing on the piston. It vents from the gas block into the stock.

Collector, the sight is not in perfect alignment with the key. I imagine the barrel might be a degree or two off but I can see where the sight seemed to be a loose fit on the key until the pin was pressed in. Whipping up a little fixture to hold it in place while I reinstall the pin should be easy.