Bringing back an old thread.
I recently moved from a Brompton to a Birdy and in researching others' experiences, came across this thread.
On the Brompton I did some touring and found the Brompton front-mount 'touring' pannier very good. I've made a simple adaptor that allows this pannier to be mounted on the Birdy. It attaches to the headstock area via two plastic hose-clamps. The pannier can be removed almost instantly (it unclips from the mount) and then is easily carried around. The bike still folds fine but is a little longer.
The pannier mounted. It looks a bit messy when empty but fills out nicely when full. Brompton recommend a max of 5kg and that seems about right. I also tried mounting the pannier on the handlebars and found, as have previous Birdy users, that it really negatively impacts on steering. Mounted as shown here the impact on steering is way less - almost negligible.
I also have chosen to fit a softer rear suspension bush. I have used a cut-down rear bump stop from the suspension of a Daewoo Matiz. It is more progresive than the original and allows more suspension movement on a given bump. I use cleated pedals and unless I am tired and am pedalling jerkily, rear suspension bounce is tiny.