I have done all of the "Caswell" gun mods to my HF guns with great results. The charge difusser lightly presses into the barrel and can be removed and modified with a pair of needle nose pliers. I have made it a habit to remove it each time I clean the gun to make sure every bit of powder can be cleaned out.
One additional note, I highly recommend using a nice regulator with pressure gauge at the gun than can be adjusted and locked down. This works WAY better than the air valve at the bottom of the gun handle. With that being said, make absolutely sure you have the air valve WFO so it becomes essentially inert in the air regulation equation. I fought a problem with air delivery for two days before I realized how stoooopid I was for not paying attention to the air valve
Now I lock it open with epoxy to make absolutely sure it doesn't get closed. Did that make any sense?