250grs TTSX will work properly in plains game!
My gunsmith used the 250grs TTSX in his .378 Wea Mag and is satisfied.
The 250grs provide a perfect match of muzzle speed and bullet weight / length.
An alternative could be the 270grs A-Frame, which I use in the 9,5x66SE (see cartidge in the middle, very similar to the .375 Ruger). Great performance on plains game.