What rated leaves do you have now??
I updated mine from stock to 250Kg's and they are bearable unloaded. I have an alloy tray and canopy, so there's not alot of weight over the rear. If you're still running stock rear leaves, and are thinking of upgrading to 300Kg leaves I'd say you would be OK when the truck is unladen.
But, you may find 300Kg leaves are still not stiff enough when loaded, you still might sag. That's the beauty of air bags, you just put more air in 'em.
I suggest you have a chat with some professionals. It's not rocket science, but experience is always a good investment.