RE: iSonic 111
9.0 will be useable but over the top. (How's that !!??)
OK, HS105 and iS111 are very different animals, but that feeling of over gearing the rig for a given rider weight/board size you already know - and don't enjoy... If you're really looking to "sweetspot" good versatile speed around 7-7.5m thru 8.5/9.0m bigger riders, have a good look at the 122, it's definitely more the bigger rider's choice if you don't want/expect too much use with sails below 7m (performance tuning/riding, not comfort !).
Not sure where/if that fits with your other boards, but definitely worth considering (noting Grant's 90kg/111 option above based heavily on sweetspoting across the 6.6-7.6 range as well as having some minimal reserve for the 8.5m).
If he was asking for 7.6m+ 8.5m, no doubt what the answer would be...
Cheers ~ Ian