As several people know, the Birdy frame is able to accept 18" ETRTO355 wheels, 16" ETRTO349 wheels of the Brompton and Pakit (only 6mm smaller than the 18" ETRTO355) and 20" ETRTO406 wheels.

I have ETRTO349 and ETRTO406 very similar wheels for my Birdy on which I mounted the same type of tires available in ETRTO349 and ETRTO406 and from my tests, the 16% bigger ETRTO406 wheels do not bring any noticeable advantage if everything else is the same.

Eventually, the ETRTO349 wheels provide the fastest configuration for the Birdy because its possible to use the very fast Greenspeed Scorcher 120 tires with them.

So from my experience, between ETRTO349, ETRTO355 and ETRTO406 (and probably ETRTO369), the wheel size is not important, what is important is the bike design and construction and the tires.