In the wsm's i have seen more consistent ES's with RL23 and temperature testing looks to be really stable in cold temps.
In the 270WSM with 150 grain bullets I would start at like 59 - 60 grains and work up to 3000 - 3025fps.
Can compare Alliants data here for various bullets, most top out in the 62 to 63 grain range. RL23 burn rate seems to be just a tad slower in the WSM cases than H4831. http://www.alliantpowder.com/resources/ ... Powder.pdf
If case full % and top speed is your desire then yep, RL26 or IMR 7977 gonna do it, but ES's will be a tad higher and temperatures will have a bit more effect on velocity.
Just bought some RL 26 for my 25-06 and it is working real well with E-Tips. Going to try it next with my 270 WSM and E-Tips or PT. Been using MagPro will 150g PT and that works pretty well. Will let you know how the RL 26 works.