I'm getting 2850 out of a 300 gr. pill from my 375 RUM and I can squeeze a little more from it.
In a 378 Weatherby you can put about 7-10 more grains of powder; maybe 12-14 grains if you "improve" the round and make it nearly straight walled. You're already at max with the 378 W. and not really going to gain much. You could make 2900 fps maybe a hair more with a 330 gr bullet but that is really stretching it.
To get where you want to go, you are going to need a couple or three tricks to get there. First is the longest action possible so you can seat the bullet out further allowing more powder in the case. Then you need lots of freebore to help with the pressures you are going to encounter. Then you will be talking in terms of 26" minimum barrel length for the powder burn.
When all is said and done you also should be talking a 10 lb.+ rifle sans scope because it's going to kick like a Clydesdale
.... trust me on this one.