Most stunt engines only pick up about 1000 revs from the four stroke to two stroke break. I would think they would have a stronger break. To break 2000 revs it would have to be piped where it also gets on the pipe when it breaks. As far as I know all high performance engines will four stroke. The reason they are not used for C/L stunt is because tipping the plane up is not enough to get them to break, or they often will not return to a four stroke mode after the initial break.