If you are in real speed conditions >20kts and flat water then you should be pretty OK (at 80kgs) to use the iS50 - and be competitive on that. Of course. as conditions "improve" for even more speed, the iS50 only gets better as it gets further into the real performance groove.
(As always) it depends on a large number of variables, including the rider's style/experience/tuning/technique/course angle (as well as weight, which you covered) - but certainly most speed riders will find the effective transition (for flat water speed) between iS87 and iS50 somewhere between 20 and 25 kts. For the record, it can be lower than that in "ideal" speed conditions (rare) ~and also to note that (especially in "rougher" conditions or bigger guys) the iS87 is certainly not limited to only 20-25 kts max for speed use.
It's very interesting to see the number of riders who tried the iS50 who found it a lot easier to get (and keep ) going for a board of that size than they had expected...
It is possible that a small (but maybe more common in some locations) performance band exists between iS87 and iS50 where riders running pure flat water speed (GPS-SS etc) look more for dedicated speed than the open water speed / slalom potential of iS87 - and yet don't have quite enough conditions (etc) to be fully into iS50 territory.
That has not escaped our attention and a logical solution is certainly in consideration.
Cheers ~ Ian