Saving weight is asked frequently. I myself have a lighter xt660z as well. But remember, is putting a lot of effort with relative little result.
Depending on what you're after, it's better to start out with a light bike and go from there.

For the exhaust, with any engine if you change the intake of fuel or air, you need to change the injection or carb.
With injection you'll need a new map or fool the standard equipment to make it squirt more or less fuel.
A new map is always better, because that is like a custom suit. A custom suit makes more sense of you're shape is more out of the ordinary

The further you're talking your mods the better a new mapping will be compared to a fooling of the standard equipment