The Lott has a larger case than the WM. All things being equal, it will give 50to 70 fps more than the .458WM. Stories about 200 fps faster are a myth. Maximum velocities in each require compressed loads (see Barnes Reloading Manual #4).
High pressure in the .458 or in the Lott is a myth with modern powders. It stems in part from the temperature sensitivity of obsolete cordite powder 100 years ago and in part from some problems that Winchester had with WW748 ball powder in the 1950s.
It is simple to get 2200 fps with a 500 grain Barnes banded solid in the .458 WM but WW 748 is not a very good powder for attempting this. Winchester uses WW748 because they make it and they're too cheap to buy some good powder. That's why their velocities are lower.
I just recently started reloading 458 Lott and I am using Hornady DGX 500 gr , 350 gr interlock FP. I tried a few of their recommended loads and was not to impressed with what I got compared to the factory stuff they sell.
When surfing the net and in the cartridges of the world reloading book along with Hog tech service , Jack Lott himself all indicate 500 gr bullet with 75-85 max gr of IMR 4320. Hornady for some reason recommends a Max of 77 gr IMR 4320 with DGX / DGS ??? with real poor velocity ratings as expected.
Have you ever shot the DGX / DGS rounds with IMR 4320 before and if so how did it work pressure wise ?
When I asked Hornady about this they just said that they used a Ruger 1-10 twist 24" and that's all they could get out of 4320. I also noticed all their recommended loads are are lower velocity then what they sell pre made. They won't give up their secrets either I tried already. They use 77 gr of something they won't share ?
Can the 500 gr hornady rounds be so different then others that IMR 4320 at 85gr is not an option because of their reload data or did they just have a bad shooting day at Hornady ?