The .257 WSM has been disappointing for me so far - it has good speed, but a very large standard deviation of velocity. At least with powders that let it achieve its full velocity potential. Might work pretty well with IMR 4350, but you might as well have a .25-06 AI. I would look at the .257 WBY or the .257 STW if you want a high performance .25 caliber.

One very good WSM wildcat is the 6.5MM. It can push a 140 Nosler Partition to over 3300 without any pressure issues and is very accurate. I hope Swift makes a Scirocco or Nosler makes an Accubond for this caliber sometime soon. Should really make this caliber sing.