If you look here you can see the ST dosen't have this bit
http://www.parkers.co.uk/cars/gallery/?model=1391&img=archive/Ford/Fiesta%20ST/DSC02213.JPGI've had a look / feel at the underside of the ST bumper and there is a lip to help mount the black bit, be it a stone guard or splitter or thingmjig. It should be the right size and shape to fit straight on, insted of trying to graft on the flipin Laguna splitter! It will make the front end look lower whatever it is!
Also saw a new Clio Cup at the weekend and was liking ther rear defuser, I know it will be 100% ineffective, but it looked good, like this
http://www.parkers.co.uk/cars/gallery/?m...-)/IMG_2543.JPGI'd stick something like this on the ST