I've been using K86 Dram for a few months, it's a bit heavy but I believe it's strong (serious wipeout/crash testing expected this autumn
). As for size, on paper it's K96 for your sails. In practice though, some of my lighter buddies tried it with a 6.5 and it was fine (I'm 85kilos so I don't go bigger than 6.0). If you're mostly on a 5.0 and occasionally on a 6.6, go for the 86.
I wouldn't know anything about it's suitability for freestyle.