RE: 120 litre speed board ????
Fully geared up I weigh about 100 kg aswell, and even with wind dropping I can get back to shore with a iSonic 111. Pulling the sail out of the water can be done but takes some good balancing. Now about the speed, I have regularly done speeds over 35 knots with the iS111 using a WarpF2006 and Select SL37cm fin. And that was not on very flat water. As soon as wind increases I get on my iSonic 87 with a WarpF2006 6.8. So as a speed board the 111 is good choice up to a certain point, it is a excellent training partner for the bigger wind days.
I think I can get iS111 close to 37-38 knots in flatter conditions.
I always post my speeds on www.gps-speedsurfing.com.
good luck with your choice!