My Brompton has been through a bunch of changes since posting this thread. I've been using an S stem with lightweight stem extension, bought from a hong kong ebay seller for around £15. It's light, under 100g and seems robust enough.
The use of a flat bar has given me more space to play with brake/grip arrangments. As you can see I'm currently trying out the Shimano BL-R550 brake lever. On the 2-finger Xposure levers the distance from lever pivot to end is 7cm or thereabouts whereas the Shimano lever is around 8, and the Brompton lever is even longer. You can see from the pic that the brompton lever is adjusted in terms of placement and reach so as to be operated with 2 fingers and the result is a dramatic increase in braking power, and it's far better than with the Xposure levers. It's still not comparable to v brake performance but it's something like half-way towards that from how things were before. The corresponding reduction in brake pad movement means this requires rims to be true and brakes well-adjusted. With an M type handlebar this arrangement isn't possible so the Xposure levers are a decent solution as they move the lever pivot point away from the hand.