I have been logging some commuting times...
Birdy with Maxxis Birdy tyre on the front, and Marathon Plus on the back, both pumped to approx 60-70psi - 60min for med-hard effort.
Birdy with Kojak on the front, Plus on the back - 57min for med-hard effort, so perhaps 5% faster.
Swift - 58min medium effort.

I find it quite hard to judge effort level on the Swift. because it is so light and nimble, I think perhaps I put in slightly more effort that what I feel.

Anyway, it is slightly surprising to see that there is actually very little difference between the Birdy with good tyres and the Swift. Where the Birdy feels a bit harder is on uphills - maybe that's why I think the Swift effort is medium and the Birdy med-hard.

So perhaps if I go the weight weenie way with the Birdy it will be as fast as the Swift. Right now I am toying with the idea of 20" 406mm wheels.