I have run many many 15's since my start in FAI Combat in 1970. I have to place the Fox products at the very bottom of my list. If you get good results with on of these, you're right up there with the few people get Wen Mac's to run.
Sure, Foxes won the 1982 World Championships, but these were specially built factory engines. Enya, OS, MVVS, Cox Conquest, Rossi, etc, etc. No need to punish yourself.
70 MPH from a standing start with three lines, a hook, and a landing gear is NOT a gimmie.