It depends on what you mean by heavyweight (90 kg, 100, or a lot more than 100 kg?), and how much float you require from your board (i.e. how easy it should be to pull the rig as opposed to waterstart).
What are you going to do with your 8.5? Are you going to use it as a highwind sail on your Formula, or should it match the next smaller board?
I think that for a 100 kg windsurfer a sensible step down from a Formula with a 10 m2 sail would be a 7 m2 sail on a 115 liter board (105 liters if more skilled). That kind of board would float you well immediately when you are moving a little, but it would of course sink when pulling the rig and standing still.