If you consider Bft 5-6 (15-25kts) as being your typical conditions, then yes, iS50 would not be so easy for a 2m/100kg rider. Even if we offered an "intermediate" size (say, around 70Lt) the better choice for very big riders in light to moderate speed wind range will still be the iS87, which is definitely a very good choice as a "larger" speedboard.
iS87 has very good broad angle top end speed, with a heathy dose of control /hi speed balance added in as well, which delivers a powerful combination of "easy speed" in the range you are considering.
Interesting thing with this board is that many people (inc a lot non Starboard) have now had the chance to actually have a blast on the iS87, and the feeling and feedback is clearly positive....
It's no submarine, but not an aircraft carrier either
Cheers ~ Ian