I have worked with a Ruger #1 in .300 win mag from 1976, and a fairly new Mark V Ultralight in .300 Roy - both rifles have fairly long throats and 26" barrels - so although different makes and vintages, they are comparable rifles each with 26" barrels and long throats. Both are very strong actions.
Cartridge OAL is of no issue in the Ruger, as it would be in a magazine rifle - and you can seat 180gr Noslers well out there, around 3.65" OAL. The Weatherby can take cartridge OAL's to 3.665" and still fit them in the magazine. I've worked mostly with 180's in the Ruger, and 165's in the Mark V, though future development will be with 180's.
Weatherby claims 3350fps with 165's, and in fact factory ammo clocked 3425 out of my rifle. I got it up to 3350 with 165's and decided that was plenty, especially since the load was well over book for Reloader 22, and I was meeting factory claimed velocities. I broke down one factory load and found it had a similar charge of an extruded powder. Case life seems satisfactory and accuracy was okay. I'll try 180 Accubonds in the future, with an eye toward auodad hunting. I think 3200-3250 is probably a realistic goal for that weight. Wby claims 3250 for their 180 factory loads. I have not tried any of it in my rifle, only the 165's.
I've actually hit .300 Weatherby claimed 180gr speeds out of the #1 .300 win mag, but pressure was clearly too high and the WW brass was expanding far too much. With 180's out of the Ruger, I think 3100 is doable with good case life and accuracy. That speed may not be achievable in a different rifle, especially one with a 24" barrel, or a std length action and magazine box. And I think I'd set the goal still lower, if the rifle was an older Springfield or Mauser variant.
The .30 cal Long Range Accubond may be a real interesting bullet to try in the future.
The short answer to the OP is I think the .300 Roy can beat the .300 Win mag by ~100-150 fps, for the a given bullet and barrel length, in rifles of comparable strength and allowable COAL. As far as which one I'd pick, it would be a matter more of how I liked the individual rifle for the task at hand.
