Not a real Brommie rider, but my Merc doesn't flex at all. In fact I'd call it rigid. It's certainly as rigid as any other bike I have. I weigh about 165 pounds (Ok - maybe 169 - I need to ride and walk more).
You're right about the brakes though. You could always swap the brakes out for the double acting caliper design that Brompton now uses. I expect it will just bolt straight on since the frames seem pretty much exact. I'm assuming that you also have put on Koolstop salmon brake blocks. They improve things a fair bit, but the rear brake is still not great to put it mildly. Another trick is to wash down the blocks and rims with that citrus degreaser. I actually had the rear wheel grab and lock once after that treatment and as you are well aware, that's an amazing difference to the normal situation. Unfortunately, they also squeal after applying it, which depending on your viewpoint can either be a nuisance since it disturbs the peace, or an asset, since even pedestrians with ipods in their ears jump out of your way like frightened rabbits.
Last edited by EvilV; 02-09-09 at 10:59 AM.