I don't know about ST150 but I have ST170 calipers on my Escort with 300mm Focus discs. It's the same procedure I followed with fitting the Mondeo calipers which from memory is drill 12mm holes in the hub and use spacers to move the caliper outwards more. People were saying 12mm spacers but I ended up using 15 or 16mm as it seemed to sit better on the disc.

So knowing that ST170 calipers fit perhaps you can then find out in the ST150 can be fitted with ST170 calipers. If so I would have thought it's possible.