Rocky Mountain,
The base 338 Excaliber case holds around 7-8 grains more powder then the 338-378 Wby case because its basically the same case only that the Excaliber is longer then both the WBy and Rigby. It also has a longer case body length.
My design relocates the shoulder position to increase the length of the case body even more over the 416 and Wby case.
As far as powder comparisions, inless we are using the same exact powder, the comparisions are not accurate because different powders weight different amounts per volume.
If you simply improved the Rigby case, the 338 Raptor should still offer around 7-8 grains more usible powder capacity. If you relocated the shoulder forward and improved it making a very short necked case, then yes, you will have similiar case capacities.
Only draw back I have seen with the 416 brass is that its designed for a very moderate pressure load and I have not seen to much brass that would support +65,000 psi in chamber pressure with usible case life. Also, its just a pain in the rear to take a 416 case down to 338 and then 30 cal as well compared to what the 338 Excaliber will be. Much more user friendly.
In my opinion, when Wby came out with the 338-378 Wby, it pretty much made the 338-416 Rigby an outdated wildcat that required much to much effort for the same performance.
The 338 Kahn which is a 338-378 Wby with a conventional 35 degree shoulder angle will easily match the numbers your listing as well as it should, nearly identical to the 338-416 Improved.
Improving the 416 really does not give you much volume increase as its pretty well improved right from the start.
Hope this clears something up.