I'm 87kg, have about the same boards like you, and had a couple of months ago the same question.
I was also doubting between IS87 / FU93 / K86.
I've decided 1 month ago to buy cheap a second hand IS87 of 2007, which I couldn't try out yet because of the winter
I'm also very curious how the board will react in 1 - 2 m wavy chop.
On flat water, I'm convinced I've made the right choice.
From the moment I can try it out, I'll let you know.
By the way, I think there is still a 2nd hand IS87 available on www.windsurfing.nl