Trex
My double rifle in 450 No2 is my favorite rifle, However You will not gain enough of a ballistic advantage to make rechambering to a 450 Nitro or a 470 worth the expense or trouble in finding ammo. I would just kep it a 458 of maybe go to a 450 Lott.
Save your money and get a double rifle
Properly loaded there are no flies on a 458, and it has the added advantage of chambering easily/readily found ammo.Save up for a double you will be glad you did.