I've had good results with two loads with the 60 grain partition, data from a 22" barrel:
38.5 grains IMR-3031 gets me 3750 fps and 1" groups
36.0 grains Varget gets me 3420 fps and 3/4" groups
I got extremely erratic results from IMR 4831; using powder mfg data I was getting crazily high velocities, and stopped testing before I got even close to the listed max. YMMV, but I recommend you avoid it.
One word of warning about any handloading for the WSSM: the case neck brass is very thick. You'll need to ream the necks down (I outside ream them down to .250 neck diameter; that's neck thickness of .0135) before sizing the brass. If you don't, the neck brass is so thick that even resizing won't bring the neck to SAAMI diameter; in addition to shaving bullets as you set them, you'll see weird pressure spikes and accuracy problems. I tried a few rounds this way and yeah, I speak from experience;-)
Other than this one outrageous flaw in the case design, I've found it to be an accurate cartridge with great velocity potential--no trouble reaching published velocities with all but two of the powders I've tried.