Hehe, I think it's an awesome idea to have fat grades! Tipping the scales at 84kg with a height of 183cm, according to the bmi scale I am overweight

Personally, I think I'm just big boned.
That being said, I have done a few hard climbs and I don't think I've come anywhere close to my personal grade limit. You can always get better and climb harder.
The only real argument I have for being heavier putting you at a disadvantage is physics. On slopers, at a certain degree of slopiness, no matter how strong you are, a lighter person who is equally strong will be able to hang on where you will just slide off, but that is really just splitting hairs.
As Ebert and some others mentioned, climbing is a very dynamic sport in the sense that everyone can experience a route differently to the next. That's why I think there's a 30 out there for everyone (you just might not find it in South Africa though). I've seen girls doing hard sequences of moves on holds that I would barely dare put my feet on to do what a tall guy would do in one move. As long as you're having fun, then it shouldn't really be an issue.