I use WST in my .45 loads, so I was interested to see if it would perform well in my 9mm loads w/ the Missouri Bullet 125gr Smallball.
The short answer is, it does. Here are data for loads at an OAL of 1.09:
3.8 grains 1002 fps
4.0 grains 1024 fps
4.2 grains 1043 fps
4.4 grains 1076 fps
These out of a Springfield Armory XD-9 4" service pistol. I'm running the 4.4 grain load as my production load w/ the Missouri Bullet Smallball.
I've also run the Hornady XTP 124gr jhp with WST; a good load is 4.7 grains which produces about 1140fps w/ an OAL of 1.100, with excellent accuracy. That is in fact my production load w/ the XTP bullet.
Finally, I've also run the Precision Black Bullet w/ WST; 4.7 grains of WST at 1.120. It also does very well.
As usual, work up your loads. I was put onto WST by another longtime reloader who gave me some start loads that worked well, then I worked up to the loads above as the best performing of various amounts and OALs.