If you get tired of it send it to me.
Best factory ammo for consistently meeting factory claimed MV: Hornady 500-gr DGX for
2140 fps from a 24" barrel.
As long as you have a SAAMI-satisfying .458 WIN MAG chamber and barrel of .458" to .459" groove
and .450" bore,
you will probably get 2060 to 2080 fps from your 20" barrel.
It could easily be handloaded back to 2140 fps with 20" barrel.
+2200 fps with 500-grainers is quite easy with the .458 WIN MAG handload in a 24" barrel,
all at 3.340" COL or less and 60,000 psi or less.
Below 20" length you start losing more MV-per-inch of length chopped.
My take on some old .425 WR data from WR compared to .458 WIN MAG new data:
My SWAG is also that with faster powder and heavier bullet you will get less loss than with slower powder and lighter bullets.
We usually use the faster powder with a lighter bullet
and the slower powder with a heavier bullet for best-practice-loads,
so it is pretty much a wash.
But if given the choice of powders for a given bullet weight, go with the faster powder for the shorter barrel.
Tim Sundles has an 18" custom rifle that shoots three fps faster than his 22" factory rifle.
2250 fps for 18" custom barrel, 2247 fps for his 22" factory barrel.
Short throat will do that, with increased pressure. Other specs can be blamed too.
His Buffalo bore ammo that he rated at 2250 fps (400-gr TSX) gave 2333 fps MV in my factory 24" Ruger No. 1.
Some Federal Premium .458 WIN MAG factory ammo does over a 100 fps faster than they claim,
both being for 24" barrels.
The .458 WIN MAG rifle loaded with factory ammo is like a box of chocolates.