Can't comment on a cat but on a heavy mono running a V6, the Viper from my experience is a great prop for speed. The downside is that it is about the worst prop for midrange economy. By changing to a Rebel propeller at some rev ranges my economy has improved by 15%. Not sure they will fit the 115's - depends on shaft length possibly as I think. Have you done WOT RPM checks and looked at engine heights.