Actually,the rum has advantage over stw same as stw has over 7mm rem. With pressures being equal,the rum gets the nod on velocity but you are going to burn EVEN MORE powder.
As someone that has owned all three,I can assure you that the 7mmRUM has a much smaller advantage over the 7mmstw,than the 7mmstw has over the 7mmremmag.My 7mmstw rifles add between 200fps and 300fps over the 7mmremmag,while the 7mmRUM adds only another 50fps to 100fps.I personally prefer the 7mmstw over the other two,but only with a barrel at least 26" long.