Offhand,
I've been reloading for over three decades, and all of the responses here seem reasonable to me. I've never tried removing the shell holder retention clip, but I fail to see how this could hurt anything. Worst-case scenario, the shell holder won't stay put.
30mag,
As for run-out, getting your seating depth tuned to your individual rifle can improve both accuracy and velocity, but as Afy mentioned, seating depth DOES affect pressure, and can do so significantly.
Also, be prepared to test fire various seating depths to see what your rifle prefers. Ideally, the bullet should be seated either just off the rifling or just barely touching the rifling, but I have two rifles that hold significantly tighter groups with bullets seated to near factory specs. Both rifles like the bullets well out of the rifling.
In my Model 700 25-06, I lose about 150 fps in velocity, but move my groups from just over MOA to about 3/4 MOA. It makes no sense to me, but that's what that particular rifle likes.
I have a Model 600 Mohawk in .222 that functions the same way.
Joyce Hornady's adage, "Every rifle is a law unto itself" holds true in my experience.
KR