I have used the RST spreader loads in both 12 & 16 ga for both hunting & (a very limited amount) sporting clays & pattern results are similar to what Flintfan reports w/ opening the choke by about one step & a very even distribution of shot with reduced center density. This is what Alex who used to work for RST told me to expect.
PolyWad spreaders open much more dramatically but in the patterns I've shot you can have very uneven coverage w/some big holes. I wouldn't want to use them @ more than 15/20 yards on any type of game bird.
I've also shot a few of 12 ga 1 oz Fiocchi Interceptors @ sporting clays but have never patterned them. They are 1300 FPS & I did not like the recoil.
