How much would you need to sink the plug to make it come out the amount you want? Two or three threads at the most? Very doable in aluminum with piloted hand tools. Do it all the time for correcting the muzzle crown on rifle barrels. Not like the engine would explode if the plug was tilted half of an RCH. Just need a pilot that threads in the plug hole and a cutter that fit the pilot. Drive it by hand or with a drill motor or drill press. A milling machine would be better still. Not much different than the tooling used to cut valve seats. The end result doesn't need to be as precisely aligned though. I'll see if I can find an extra plug and see what my ProMaxx look like. Probably the same. I think that removing the excess threads in the chamber and smoothing everything out will only help things, or do no harm other than the volume added to the chambers. Unshroud the valves while you are at it.
