I am not sure about the benefit of a 20"/ETRTO406 Brompton vs. its original ETRTO349 wheels.
If it is to only being able to use relatively narrow tires 28mm or 32mm max) it won't improve the comfort nor the performances (I have a Birdy with 3 wheel sizes, ETRTO355, ETRTO349 and ETRTO406 and there is no real difference in performances with the same tires). What would increase the comfort and performances on bad roads is having wide balloon tires like the 50mm Big Apple or 55mm Big Ben. But using 50x406 or 55x406 wheels would increase the total wheel diameter a lot (57mm for the wheel size increase + 30 or 40mm for the tire width increase).
But increasing the wheel size of the Brompton increases significantly its folded size (which isn't the case with the Birdy because on the Brompton, the rear wheel folds under the frame while on the Birdy the rear wheel folds beside the main frame) and the major strong point of the Brompton is its small folded size which is lost when increasing the wheel size.