There's a fair amount of tolerance and generalisation in the name of simplicity in those sail size ranges.
One of the main variables: Rider weight; the HEAVIER the rider, the smaller the maximum sail size.
(yes, that's correct. Smaller sail MAX for bigger rider on same size board) if you want to stay within an equivalent sweetspot.
Rider+ rig weight/s (both static and dynamic) act to "overpower" a board's size (both its' volume and planing lift). A large gruntier heavy (in both nett weight and ride/feel terms) sail of xm2 with a heavy weight rider will be less "sweet" on the same board than a light weight rider with large"light" sail [of the same xm2].
Bottom line ; Unless you're into the heayweight category, the OD9.7 will be at the workable "limit" of good upper sail size fun tuning for the Fu133. Doable but not dream. The other question you might want to consider in a no BS appraisal is what is your next sail size smaller (or, how much EXTRA bottom end will you get from going to the very biggest sail size over what you have already (?unknown in this discussion
?) . In real world usage, the closer you get to the max sail size for a given board quite often this becomes a law of diminishing return. ie ; how much more PRACTICAL RANGE will you get from a 9.5m over (say) a 8.5m?)
To get the best when maxing sail size vs board, you will need to fin up accordingly to suit !
Cheers ~ Ian