I haven't used WSF but I have used a bunch of WST in cast bullet 9mm loads. I wouldn't use it with jacketed bullets in 9mm. There are far better choices, WSF being one of them. WST shoots very soft, burns clean, is very low smoke and meters great. It's my #1 cast bullet powder for 9mm, .45ACP, and .40S&W as soon as my USPSA Limited gun gets built.
WST is a lot faster than WSF, so WSF may not be that good with cast bullets.
Hopefully WSF shares some of the traits of WST because it's excellent!
I have had xlnt results in 45acp and 9mm using W231 in a variety of bullet weights. Only negative is it is a bit on the dirty side compared to others but I can put up with a bit of smoke for accurate results. As for WSF, yes, I have used it in 40SW and 357SIG with great results. Cant say about 9mm tho.