I wouldn't say boards are/were designed around 75kg sailor as the target buyer, especially not iSonics. If anything, *board slalom pros are all heavy. That's a good question you're asking, but we all have to make compromises: imagine every board line having (for example) a 100l early planer for lightweights, and then the same 100l model, but control oriented as a highwind heavyweight's tool. You simply can't have everything in perfect proportion (volume, width, etc) across all weight groups.
Regarding your iS choice, I'd say go for iS111 and accept it's a sinker. No way you will like 122 (I use it and like it very much) in coastal conditions with a 6.5 sail.
PS Do I see departure from freeride shapes, Floyd using a full on slalom? ;-)))