Bonnet weights depend on the spec really. Typical Carbon bonnets that I have seen in Japan come in around 7kgs but these have all OEM hinge fixings included as well as a bonnet catch and probably have some legal requirements there. Many I have seen here in the UK will not have the front hinge and rely on bonnet pins. They are also not required to have much structiral integrity being used for rally, so I expect the ones in Japan are built pretty heavy if a stock bonnet is 12kgs.
I am chasing down some GRP bonnets here in the UK which are claimed to be 5kgs but I have yet to receive any details. Again for motorsports use in mind.
Matt Humphris sells carbon bonnets but they are mega money, you're talking $1900 a pop plus shipping, weight is a miniscule 2kgs. I picked one up once when I was at his place and I nearly flung it into the ceiling, almost cost me I can tell you. Made to a rediculous level of quality, double skin, very stiff too....but you have to ask yourself if it's really worth it.
Going by weight, you'd be better off dumping the stock battery and fitting a race battery for like $200 and saving 10kgs that way, (CG engines need bollock all power to crank over), add less fuel to your car or cutting off a leg, (bit drastic)
Better still, drop the weight from the flywheel, makes the biggest difference in terms of performance compared to any weight reduction in the long run.
