I have a Suzuki 2.5 on the back of my 21SC. It's "sweet-spot" is right around 4.5-5 mph which at less than quarter throttle. You get 7 mph at half throttle and on a really flat day with no mast and rudders up you can top off at 12 mph at full throttle.
I suspect the 15-20 mph listed in the advert is a little bit of an embellishment. Maybe with a 9.9 to 15hp?