Agrelon. I'm using a iS144 with 9m, 10m and 11m. 51cm and 57cm fins. I'm 105kg and 194cm. I guess this will be similar in sail carrying ability to the iS131 since there's only 8 litres difference in volume.
On the iSXXL, an 11m gives no real extra "early planing" benefit c/w 10m. This is due to the extra weight drag etc. 10m is the optimal max sail size for iSXXL. It'll work, but not to any advantage. Slogging with an 11m is no fun either. For me, the iS144 is best with a 9m/51cm fin. Light wind heaven
If your looking to get out on an 11m pretty often, but dont want to go full formula, take a look at the iS150. I once borrowed an iS150 from my local shop. I was very impressed with it. It does feel a lot bigger than the iS144 so will comfortably take an 11m, but I think it will be to big for your 7.5m. 9m would be the smallest real size for the iS144.
Even though SB state 6.5m to 10.5m for the iS131, its real usefull range is 9m to 10m (maybe 8.5 to 10m). I think you struggle to find a board that will truly work in the 7.5m to 11m range.