I'd assumed the BSR was the same weight as an S-RF3, but the bromptontalk wiki says this:
2010-Present (2013) Brompton Standard Range (BSR) Hub, (range 178%), 148mm axle
This hub was designed by Brompton itself in an attempt to create a noticeably lighter hub than the Sturmey Archer SRF3
The design makes use of various standard SA-parts used in original SA-hubs, its inners are interchangeable with the Sturmey Archer SRF3
The hub is produced specially for Brompton by Sun-Race/Sturmey-Archer .
So the internals are the same, but somehow the hub is significantly lighter? The shell is already alloy, I don't see what scope there is for weight saving. If anyone can weigh a BSR hub I'd be curious to know the result. Actually I suppose interchangeable inners doesn't mean they're the same, perhaps they shaved a bit off the planet cage.
In that case the 5 speed will be perhaps nearly identical in weight to the 6.