Brompton is easier to carry over stairs when it is not folded, than folded, but you then use both hands to grab it. It is best to have the rest of any load on your back as counterbalance. Within this perspective, my most important investment into Brompton's portability was a handle for the main tube.

The market of light parts for Brompton is a swamp. Many of those parts are next to unusable. They are purchased by lovers of their Bromptons to adorn them with jewelry, show them to friends and shoot photos on a scale for FB. As a minimum I suggest strongly lowering the expectations regarding the possibilities in lowering the weight with the aftermarket parts. Getting a front derailleur for a hilly area makes much sense but making it work well requires mechanical skills.