The hitches for the Fiesta ST seem to be more reasonable in weight than some for other cars like the C6 Vette hitches that were around 30 lbs and grossly overbuilt. I made my own hitch and had it inspected at a master of custom hitches shop owner and he offered me a job on the stop, it weighed 6 lbs.
It was tucked up high and had a long ball mount bar so when not installed you could not see the hitch unless bending way, down, close and low and then barely. The bar was a bit heavy but it was only used when towing, pop it off at the track was the idea. I was asked to reproduce them but it was designed around the Corsa Sport exhaust I had, to many different exhausts used to make it viable and the liability issues were too much to deal with in our law suit filled world.
If I needed a hitch for the ST I would absolutely make my own and most likely under 10lbs in steel. I would take a look at using Aluminum for it as well, sometimes aluminum is the wrong material for some applications as having to use much thicker gauge would not save much weight if any. Eitehr way I would have it inspected again of course.
Being the car is so forward weight biased having a 20 lb hitch on the back is not a killer in all out handling if even much of a detriment or might help a bit, more likely to effect acceleration and braking results.