Hi Greg,
I am around the 70-72 kgs mark. I have found that when on slalom gear I am one of the first to get planning and usually am fairly quick, but when it gets a bit overpowering the heavier sailors come into their own, as they seem to be able to hold the rig down just that bit easier. I know a lot of this has to do with technique and have tried some of this myself and it does work in keeping the board on the water and not lifting off when overpowered. But it still hard work.
Most good speed or slalom/speed sailors are heavier than you or I. Olympic sailors are usually between 65-80 kgs in the sailboard class (men). No real heavyweights there.
If in the waves, your weight seems less important as you are not worried about terminal speed or point to point sailing. It more about the ride.
As I am near your age I can sympathise. I too have had trouble gaining weight, but that is not a bad thing as your health is so important as you age gracefully/disgracefully.
Here's to sailing for many more days.