If the gain in water capacity is ineed 4 grains or so, then the potential muzzle velocity, at the same pressure, is 1%.
JB
I got the 4gr number by the simple expedient of inserting a fired primer into a new (unfired) 300RUM case and filling it with water. I than took a freshly formed (1x fired) 300 Tomahawk case and filled it with water.
Both filled cases were than weighed (both cases came from the same lot of 100 Rem cases).
As far as "worth it", well as one wildcatter put it "it's like lipstick on a woman, it may not change much, but it's sure nicer to look at". Besides, when your starting with a new tube, and have to purchase the reamer anyway, the difference in cost is trivial to the cost of the total project