When you stuff all that powder into a RUM it's going to go faster, it had better. But the queston should be how much powder does one use to make all rounds equal say for the sake of argument 3100 fps and you have a 3200 fps RUM that uses 10 grs or more ( alot of powder)to gain 100 feet. That's the dirty little secret handloaders fail to tell when they post.
Wow!A larger case and more powder increases the velocity,what a concept.Now everyone knows the secret.
So the RUM burns 10gr more powder to gain 100fps to 125fps over the 300wby,big deal.Is the cost of that extra 10gr of powder going to break your budget?Considering that my 300 RUM handloads cost less than somefactory loads for cartridges like the 30-06,I couldn't care less if I burn a little more powder.If you are so concerned about efficiency,shoot a 30-30 and live with the limitations that it's ballistics demand,and let those nastyhandloaders practisetheir dirty little hobby.
Then again maybe some day you might actuallybe able to understand theconcept of handloading,and you might even get over your fear of those big bad RUMs.