My general conclusion is that it's very frustrating that hub manufacturers haven't focussed on improving the 5 speed designs that have been around for a long time, and which could cater to the requirements of folding bikes perfectly. The medium ratio Sturmey Archer hub never worked as well as it ought to and the current version has overly large jumps between gears. The Sachs/Sram range of hub gears are internally very precise and as a result the internals are very light (lighter than SA equivalents), but since their intended use was cargo/dutch style bikes they stuck with a heavy steel shell. The T3 is still only around 930g.
So Brompton could have worked with SRAM to produce something similar to the P5, which has evenly spread ratios and which with an alloy shell could be very light. There are options for manufacturers to create lightweight and extremely efficient hubs for folding bikes that have more than 3 gears but so far it's not happening, which is a PITA, since the typical hub weight of 1.5kg or so for a nexus or alfine is really far from ideal for something that needs carrying.