Those Braille Batteries are great, they actually tip the scales just over 11 pounds. I have an Odyssey 680 which is just around 15 pounds in my MR2 Spyder. We sell both batteries; the Odyssey is cheaper but not quite as scant.
Braille/Recaro also offers a fixed back seat customized to your dimensions and cloth/color preferances. These dont weigh much more than their batteries.
I removed my entire front trunk and plastic cladding in efforts to cut the weight.
The Spyder is about as stable on the freeway as a cigar boat on the Bering Sea...
Next time I'll just hit the treadmill.
PS: Has anyone weighed a Boxster 986/987 Seat?