I think the twist of the stock 450bm 1:24 isnt low enough to stabilize the really heavy projectiles either. 300grain is probably out of the suitable window,
So even though there is better combustion, there are still some diminishing returns.
To improve your accuracy, make sure you are shooting hardness over brinell 12, gas checked and sized appropriately 0.001" over bore size.
The best REPEATABLE groups have seen with 300grain jacketed is right around 1.5" .
I am not the greatest shot and this weight has been very forgiving - for me this is very adequate,
from my understanding, the 458 socom has a 1:14 twist, and 405grain hard cast gas checked is netting no more than 2 moa, at 1400-1700 fps.
if you want accuracy out of the 300 grainers then try the magnum jacketed projectiles, which have recently hit the market again after drying up for over half a year