1) I've a few 788's,they are fair to middlin'. They typically shoot very well,but what Remington doesn't?

The rear locking bolt lugs,are a concession,for high round count forays. They can also be hard on brass,due to some flex.

Weighing what a new barrel costs,I'd cut my losses,let it go for whatever I could get out of it and then upgrade(twin locking lug Remmie),less ever looking back.

2) The .458" bore is a niche that though very well supported in projectile selection,it doesn't have any high BC Mojo going on for it.

The 500gr X has a BC of .526 and that is the widest window of opportunity,that I'm privvy too(.375's is the upper echelon of my interests).

If you wished to cast your own,you could spec a mould of your own design and go that route.