Kirby - Not sure where you are getting your A-Square Excalibur brass prices from as I purchased 100 cases directly from A-Square about a year ago and paid $4.00 a case. I just checked A-Square's website and the current cost of Excalibur brass is $40.25/10, $4.02 a case. Not sure where you found it for $6-$8 a case. I would agree that the A-Square brass leaves a lot to be desired and am glad to hear that Jamison is now offering it.
RM - I think you'll find the difference in case capacity between the Excalibur and 338-416 Imp lies in the length of the body. While one could use 338-378 Weatherby brass to make Excalibur, it would end up being shorter in length than the Excalibur case and the belt would have to be turned off. In short, there isn't an easy way to make Excalibur brass from another. Diameter at shoulder 0.566", length from head to shoulder/body junction 2.450", length to shoulder/neck junction 2.633", overall length of case 2.990".
My father and I both have 338 Excalibur rifles. My father's is built around a Ruger #1 in order to go with a longer barrel with muzzle brake
and still have a cartable hunting rifle. He shoots 275 gr. Swift A-Frame bullets at 3025 fps from a 27.5" barrel. Mine is built around the Lawton 8500 action, single shot, right bolt/left port. Barrel is 36" plus custom muzzle brake. I'm shooting 300 gr. SMK at a little over 3000 fps.