In the test done a number of years ago by the American Rifleman, the increase in useful barrel life was something like 15 - 20% more in the .308 Winchester if I recall correctly. May not be as much in the .300 Ultra Mag class of cartridge
Since the powders used in the 308win offer a very different burning rate than those used in the 300ultramag and the case capacities are very different(roughly twice as large in the ultramag case) how do you know that the example you offer would be at all applicable concerning the 300ultramag?