The new Futura101 will carry a bigger sail loading (slightly) better than the ST104, thanks mainly to the extra overall (about +5cm) and tail (+4cm) widths of Futura.
And if you love the character of the ST104 you will be rather happy with the FU101.
But realistically, in both cases you are very much at the top end (sail size) of the sweetspot range for both boards, and making a change to the FU101 will not provide a quantum improvement in 7.5m handling (especially for heavier riders at ~90kg).
So recommending a board change simply for that improvement on it's own (in these specific and marginal conditions) would be a pretty debateable suggestion.
Making an upgrade to the new FU101 to take advantage of the overall performance mix, while still having a good resale opportunity on ST104 could also be more worthwhile idea.
Also worth noting that both the V8 and RS sails can deliver fairly hi dynamic loadings onto the board/s for a given sail size/area (say in comparison to a 7.2m no cam freeride).
Adding a small amount of extra fin on the existing ST will possibly help a little, more so in marginal ST104+7.2m conditions, but again will not be a significant improvement over your current setup.
Cheers ~ Ian