I'm also 87 kg.
From 12 - 15 knots, I use a 125L board with sails 7.5 & 9.
I'm a recreational surfer, but with a combo like this in those conditions, I don't see many guys passing me.
I find that once the wind drops below 12 knots, I really can use all the volume I can get.
In these conditions with us at the North Sea, the wind is also very often not too stable anymore.
It often happens that I start sailing with 11 knots while half an hour later only 8 knots are left. All averages of course.
To be quite honest, in these conditions, I'm not interested anymore in speed BUT more in keeping the board planing comfortably.
The board that helps me the best with this is my SB F161 with 11m sail.
It helps me pointing when the wind gets a little stronger to recover from down wind racks when te wind gets a little weaker.
Again, planing in these conditions is really what matters to me the most.
If you say that you dont really like Formula equipment, then it seems to me from the experiences of Mark H, that the IS150 is your best option.
I'm actually also curious to try that board one time and compare it with my F161.
On the other hand, I dont think I need anything in between my 125l board and the F161.