There are certainly no flies on the .300 WinMag when shot in a single shot target rifle. When I was a member of the USMCR rifle team back in the 1960's that is what we used. However, 1000 yard target rifles don't have to feed rounds through the magazine, and generally don't even have magazines, so the bullets can be loaded out as far as necessary.

According to The Handloaders Manual of Cartridge Conversions the .30-.338 has a case capacity of 84.41 grains of water compared to 85.78 garins for the .308 Norma Magnum and 90.36 for the .300 WinMag. Any advantage gained by the 6 grains of water difference would be lost by deep seating the bullet.