Many people think the that three to six speeds is all that is necessary on a folding bike, but I beg to differ. I installed a Shimano XT/XTR 10-speed drive line on my old Birdy, and found it to be a revelation. I could climb the steepest hills without getting out of of the seat, and steep descents were no problem either. The only issue with my kit was that the end of the medium-cage derailleur came a little close to the ground. This would be a bigger issue with a Brompton, but would probably be okay with a shorter cage.
I have not tried to modify my Brompton yet. I have widened the triangle on fullsize steel road bikes without any issues, steel is not so difficult to bend. A fair amount of cold forming is done to align the various components before a bike is assembled, so it shouldn't be a big issue. I used a threaded shaft with nuts and washers to spread the triangle, and a Park tool to measure the distance from the axle flange to the wheel at all points to make sure the bend was flat. On a shorter triangle (like on a Brompton) the dropouts will bend outward as the triangle is opened, this will have to be corrected.
I would consider using a Nexxus internal hub, but I find they don't shift as smoothly as the Sturmey Archer units. If I were to convert, I would probably add a derailleur hanger and a 9-11 speed driveline, with rapidfire shifters.