The SL9 is just a upgrade from SL7 for serious speedsurfers/ slalomsurfers that are looking for better manouvrebility, gybing. Still I have only tried a SL9 33cm proto, and it is also powerfull.
If you want to beat a SL4 fin with a select I would recommend for control a S07 M, that is my favorite control fin. If the water is REALLLY a mess the select devil is the way to go in smaller sizes, but I only take the devil with deep downwind, the S07 M is giving me enough control in 99% of all conditions I sail. I like the topspeed of the S07 M better.