I fitted my Birdy with an XT 10 speed freewheel, medium-cage derailleur, and shifters, using the stock hub. The freewheel is 32 teeth, which might seem excessive, but I can climb the steepest hills in the area without standing on the pedals.

The XT 10 speed conversion was completely bolt-on, no tinkering was necessary to get it to work. Upshifts and downshifts are flawless. The only issue is ground clearance in some gears, but the derailleur seldom takes a hit, and the XT takes hits without damage, or even getting knocked out of adjustment.

Unfortunately, my Birdy was stolen, so I am now on a three speed Brompton, and am missing the flexibility of the Birdy quite a lot.