I thought the swytch was about 1.5kg and the newer battery is supposed to be more compact and lighter. Either way, if you go for that, check the crankset/BB compatibility before you swap for a lighter hollow tech system.
Also, the added torque and stress from the swytch kit is likely to be OK with standard QC/QA brompton parts, it might not be the case with lighter 3rd party components that may not have the same QC/QA policy as Brompton of any other know manufacturers.