I use a 10m Point 7 AC1, it only uses a 520 mast so it feels loads lighter than 11m formula sails than tend to use 550 masts.To be honest it doesn't plane that much earlier than my 8.6m Severne Turbo which I truly love using on those days where there are holes in the wind. My board is the 2013 model but the 2011/12 are just as good. I used to own a formula board and boy did it plane early and it flew upwind but it just isn't fun like the US. I'm sure you would get a couple of knots more out of an isonic but you would fiind the US easier to gybe, I am 86 Kg's. No idea of lower wind speed to get it planning but my guess is around 9-10 knots.
Can't you go and demo one? Honestly if you are not racing get a US