The Speed P8 is a bit smoother ride than the Mu but by far the biggest difference is made by the tires. Swap out the Marathon Racer tires on the Mu for a set of Big Apples like the Speed and you almost won't be able to tell. Of course, the Big Apples are heavier and slower too. You could also put on some Greenspeed Scorchers and get fast sticky tires which are in the Big Apple comfort range.

If you're in NYC you must also go to BFold (http://bfold.com/). This person knows folding bikes better than anyone else in the city. He can also give you an opportunity to try a Bike Friday Tikit, a Mezzo D10, and a Swift. The Tikit and Mezzo have the same size wheels as the Brompton so they'll be bumpier than a P8. But they can also be fitted with Scorchers which I think makes a big difference. The Tikit will also be the answer to all of your reach issues (it's highly customizable and comes in three sizes).