+1 - having ridden a few bikes with small wheels [16" & 20"] through the centre of town with all the crap roads and debris you find in any large town I can say that I would not qualify the ride as abysmal...in fact I often ride my small wheel bikes preferentially because I enjoy the ride so much.
Adding suspension to a Brompton is really a waste of time IMHO! I have a Brompton 6-spd and it is truly a wonderful piece of engineering for what it was really designed for. (i.e. commuting and using public transport) For longer rides I prefer my Velotechnik Grasshopper-- only slightly bigger wheels (20") but recumbent with front and rear suspension. It also has a Rohloff hub and disk brakes. I find the biggest danger when returning to the Brompton is the comparatively deficient brakes! Although the Grasshopper folds in half it's no Brompton in terms of folded size (L96xW60xH70 cm) It all comes down to "horses for courses".