Shims only help if the gun tries to fire, doesn't release any air, and then can't reset. This only happens if the bolt encounters a blockage directly in front of the bolt. Most of the time, even when that happens, the bolt moves far enough to expose the vent hole anyway. Shims only change the distance that the bolt needs to travel before the bolt stem can vent air and reset. So, if you are getting occasional bolt stick under normal conditions, it is most likely a carrier related issue.
What are you using for an on-off assembly?
The blue bumper is fine. It was designed for the classic valve. The softer one will get shreaded anyway unless you epoxy it to the front of the valve. The fronts of the valves are the same dimensions and material as on the retro valves and I haven't heard of anyone having issues with any classic valve with a blue bumper, even after 25 years.
Except for the Automag in front, its usually the man behind the equipment that counts.