I don't know about mag well conversions to inclrease COAL for your rifle. There are some conversions for Rem 700's and Win M70. e.g. Wyatt's Outdoor Custom Rifles - Magazine Boxes
I've seen where Jim See (Coyboy in these forums) at installs them.
As for seating depth, I have seen a few situations where a bullet may shoot well at different seating depths. I'm not familiar with your bullets. But, Berger published a paper relative to theirs and I've had similar discussions with Swift about Scirroccos. (The latter was of no help because my twist was too slow.)
If you don't want to change bullets, you may want to try shooting some groups at .040", .080" and .120" jump to see if you can find another accuracy node. Then, you could fine tune the seating depth and powder charge iteratively by changing only one thing at a time.