Re: Curious about the 7mm RUM..
Regarding the overbore question, it really is dependent on the powder you use. We're very fortunate to have such a variety these days. From my perspective, with a 30" barrel, the optimum powder burn speeds fall between kinda slow (RE25, MRP-2, IMR7828) and the really slow (50BMGs, etc.) with the median leaning toward the REALLY slow.
A longer barrel is an obvious help, since you're burning 85-95 gr of powder through a .284" bore. A short barrel & moderately slow powder would lead to a fireball of powder burning OUTside the muzzle & not upping the velocity. The RUM has the advantage (as Jerry pointed out) in that heavier bullets are more efficiently driven by it over an STW. Mine is driving 176 gr projectiles at just under 3,400 fps.
I had to modify my case trimmer & buy a Redding Ultramag press due to the case/ OAL cartridge length with VLD bullets. Small things really, but they add up to be noticable.
All-in-all, it's a performer - with a price.