Hey SS I had a thought - I remember someone, and I think it was Kevin_M4, who had a loose gas block on their Saiga conversion, and silver soldered it onto the barrel. That may be a quick-and-dirty way to help your loose gas block issue.
The fact that it bleeds gas around the port will definitely cause lower barrel velocity, obviously resulting in less energy to cycle the weapon.
Another thought, and I learned this from my MG42 build, and that is to degrease everything, use a sharpie or two and cover all the moving parts in black, and cycle the action by hand. The chafe marks are where rubbing is taking place, and in theory there should be very few places that rub. I found several areas that rub using this method. The areas were small, and I ignored them until I polished them one by one, and the net effect was a smoother action.
Hopefully this helps out. The piston size should still be considered as a to-do item, but if you do these things first it may not matter.