A good resource for wheel weights:
http://www.wheelweights.net/
here is a picture of the lightest 'normal' sized wheel:
http://machiii.net/images/wheels/volk/big_CE28N.jpg
I don't think the body kit will effect mileage that much at all. Negligible weight addition, as well as drag coefficient increase. It would reduce lift and underbody turbulence, but we have panels for that anyway ... but also if you are driving fast enough that a little more surface area is affecting mileage, your mileage at that point isn't great anyway. Increasing tire pressure would counteract most of what you lose from air drag. Those ultra-lightweight wheels would also counter the body panel weight addition, and if you can drop a few pounds
As far as mileage extremists ... I am waiting for someone to completely strip out their car and all unecessary components ... backseats, passenger seat, interior trim, windshield washer fluid, repace doors and body panels with carbon fiber ... hehe I have done it on a sports car for better handling and quicker acceleration, dropped about 1000 lbs from the car ... I bet the same can be done for the Honda Hybrids. If I didn't love all the luxurious feel I would consider it, but I like my creature comforts on this car.