Chuck, Kevin's an excellent gun smith, this, according to my friends on a small bore forum who like to use him a lot.

Hornady's manual lists 2,800 with 270's and 2,700fps with 300's, from a 24" WTBY Mk IV in the .375 WTBY magnum.

I've seen other data for 270gr. as high as 2,900fps, but 300's reached only 2,750fps from the same rifle. Much depends on the bullet's bearing surface and composition when producing pressures, along with how individual rifles handle pressure. Some are fast, while others slow and everything in between.