Sized up the ST185 discs today and :D is all I can say in comment...
here's some pics.
The difference in size of the discs
Here is the spacing around the caliper bracket, as you can see it's pretty centered as it is
The disc was only held down by 1 bolt, but the disc was flush with the hub and the bolt on caliper side so this is a pretty real measure.
And this is the finished product... at least how it will look once the discs are finished.
No spacers required which is fantastic, it all just bolts up.
Now to get the discs drilled to 4x100 and slotted and I'll be a happy chappy.
I'll also like to comment that my car is running so awesome at the moment.
Since I replaced the front suspension struts and put the swaybars in it's made the car feel fantastic to drive on a nice twisty road.