I use a delrin sealing washer about 1.5 or 2mm thick in place of the C3 bumper. It makes a little more mechanical noise but gives a little more velocity and durability.
When using more hammer weight it's important to finish your tune on the valve stop to keep from wasting air. Extra hammer weight isn't necessary under 150 bar.
Before elongating the barrel transfer port you need to chamber a pellet to be sure not to disturb that area.
Yes Heavy Impact. Good point. Mark how far the probe goes into the barrel carefully. There isn’t a lot of material to enlarge the the hole in the probe in a forward direction. You don’t want to inlarge the TP in your barrel to find you’ve gone to far beyond what’s available on the probe. They need to be done together.