I am 99.999999% sure the ST rear spoiler and European spoiler from the Ford Accessories are the same. I would see no reason to develop a spoiler that is different. That would mean the rear Mountune extensions will work. There sure isn't a lot of difference between stock and the Mountune front spoiler thingie. It does bring the surface in front of the intercooler out to the rest of the bumper, so that is nice.
I do have some pictures of both the ST with my 2011 Fiesta, but have not posted any of them yet.