The Carve 145 wasn't slow. It used to win a few slalom races some years ago. And that's one of my points. Modern freerideboards aren't slow or low tech at all. I doubt most riders would be able to squeeze more than 10% more speed out of a slalom board compared to a good freeride board, but those 10% make the whole difference on the racing course. It's not like comparing a family car to a formula 1 racer.
Regarding the iSonic 122 I use a Tushingham Lightning freerace sail, give it plenty of downhaul, loosen it a little on the boom, straps back and out, mast foot in the middle.
Nice, fast and controlled set up and I can really max it - no problem it's a lovely board, but I haven't cranked 30 knots yet ( I sail in quite choppy conditions) as I did on my Carve (which I sailed a LOT!!). Probably it's a matter of time on the water.
It will come though, and I believe the iSonic is capable of +33 knots with my ability.