JPK
Sounds like when you get the iron sights on the 375 zeroed you will be good to go.
I would not worry too much about shooting the 458 from the bench. Shoot it standing at 15 25 35 and 50 yards, and shoot it kneeling and sitting at 100. There are no benches in the field.
If you remember I have handled your Thys 458 when I was at Champlins. I like it and think it is a nice double.
As it seems to be working great, I do not think I would rebore it either.
What are the details on the loads JJ worked up for the 458?
I have taken several deer pigs and coyotes with my 450 No2 double and 350 Hornady RN bullets.