Wanting to upgrade my fins (set on a thruster set-up) for my Vex Profusion 8'0 (8'0x29=115) I weight 79kg dry. I mainly use the board in larger swell 4-6/7/8ft. I was thinking Shapers S9s (5.19") but did not want to kill the board am worried that it will not be as loose. Was thinking 4.7"s rather but hard to find in futures. Any recommendations? Will the S9s work well without stiffening the board up to much? Cheers in advance.