Spot on about Tom Siatos - remember now more clearly. I'd think it would produce in the 2,300 to 2,350 fps range with 500's.
I achieved 2,200fps in my little 2.1" .458 Alaskan using .348 brass.
If it fed well, it'd be a great stopping, magazine rifle.
The white line spacer is rather nasty - glad there isn't one on the grip as well, but surprised there isn't. An easy fix on the butt to a solid pad.

Nice piece of walnut, too.