Strictly speaking the BWR system is heavier, you have to include the shifter, cable and extra derailleur bits. That's probably 200g. The total is still light but heavier than a SA 5 speed hub. The big problem with the brompton drivetrain is that the tensioner introduces quite clear inefficiency, so it seems wasteful not to utilise it as a derailleur. And of course if one were to install a 5 speed hub the result would be a ten speed system lighter than an alfine.
I still haven't ridden a 6 speed Brompton, I really need to see how it works in practise as I have trouble believing it doesn't have quite real flaws.