Huw,
After re reading your post I realised your were asking a question , not making a statement. So I would like to add to my previous response.
Too heavy? I think not.
The F16 box rule allows a mast up to 9m high and the sail plan as well as sail shape can be invididually design for your body weight.
Couple this with the use of a kite I think you will find crew weight to have much less impact in F16 (solo) than other classes.
Personally, with the added flexability in the sail plan, I think the heavier guys (cat rigged) may actually be in a better position to have an edge.
Only time will tell.