I think because the 8'3 keeps the 32" width of the 8'5 and only loses 10L in volume it's going to be almost as stable for you at 82kg as the 8'5 would be, it's just going to sit slightly lower in the water which may help you anyway in choppy seas.
I'm 82kg and found the water was just about deck level when on the 8'3 and I wouldn't want anymore volume.
Also, you're 5'9, I'm 5'11, and I found the 8'3 plenty stable for all sea conditions, so I think being two inches shorter may mean you would be fine on the 8'3, also buying the smaller board may help you lose that 6kg
I'm a big allwave fan & have had the 8'10 then the 8'6, now an 8'3 is on the wish list

Demo if you can to be sure!