That's interesting. My .458 2" gave identical vel. using identical loads of H4198 and Re#7 with 400gr. Barnes spitzers.

Do you feel increasing the load of 4198 with the 180gr. will over-stress the rifle?

I think you noted 1 1/2" between centers of the loads you've tried. Is this at 25 yards or 50yards.
How far apart are the muzzle centers?