I can not help you with specific data. My brother shoots a 7mm STW AI, Nosler 140gr BT at 3500 fps. It shoots into the .3's. He is using either H1000 or RL-25. not sure which one. He has messed around with IMR-7828 too.
I would think that you will find that you will get your best results with the ultra-slow burning powders. There are 3 loads listed for the 6.5 STW on www.reloadersnest.com
all of which use very slow burning powders. I also see that they have an extensive list of loads for the 257 STW. I would leverage off them as starters and go from there.
Get a accurate micrometer and a good cronograph. We stop when we see 0.001" of case head expansion, and then back off 1.0 to 1.5 grs of powder and call that maximum when loading for a wildcat. I like to use those New Hodgon Extreme powders, they tend to be more stable over temperature.
Proceed very carefully and pay very close attention to what your cronograph and micrometer are telling you. It might be smart to invest in a bullet puller too. That way if you go to hot, you can pull the bullets, dump the powder and back off.