Sorry not to get back to the latest questions sooner, but was gone huntin'.
Canadian Lefty, according to my calculations with case capacity, the .300 Weatherby should indeed get about 3.5% more muzzle velocity than the .300 Winchester, in equal barrel lengths at the same pressures. This means that if we can get 3100 out of the .300 Winchester in a 24" barrel, we can get around 3208 out of the .300 Weatherby in a .24" barrel.
Note that this does not mean with the same powder, but whatever powder gives top velocity in each case. Also, it wouldn't quite the same if the standard throating was changed in either round. For instance, you can get a little more out of a .300 Winchester in a longer magazine rifle, with a chamber throat longer than standard. But it still will never match a .300 Weatherby, everything else being equal.
This is also an approximate number, but you will be able to get about a 3.5% increase with any bullet weight.
