I have done 2 commutes with the HRM... not enough to get some averages, but enough to get an idea.
I felt very similar the 2 days, and the weather was very similar as well. I only clock time in to work, then the wind is very low.
I also pumped both the Swift and the Birdy to 100psi. The Swift has a Conti GP at the front, and a Primo Comet Kevlar with a Slime liner on the back.
The Birdy, weighed this morning sans luggage, is 12.6kg. I must guess the Swift at 9.5kg sans luggage (both cases luggage is a backpack).
So, the times were 56min for the Swift, 57min for the Birdy. Both cases the HRav was 129bpm. In the Birdy run, the HRmax was 213 so it picked up some interference somewhere, which might alter the average a bit, if anything it might then be lower without the interference. the Swift max was 167bpm, that is probably real.
I think the 1 minute difference can easily be explained by the extra mass for the Birdy, since there is a not insignificant amount of uphill. There was also 1kg extra luggage on the Birdy in the backpack. Plus the Swift gearing is more suitable, and the derailer runs slightly more efficiently. Plus on the Swift run, there was another roadie cyclist in front of me for a few minutes which often has the effect of putting in more effort, although I tried not to. That may have made a few seconds difference.
The Birdy sure is a whole lot more comfortable. The Swift sure feels a whole lot faster.
Seems to me that tiny difference in speed (less than 2%!) is not worth it on the Swift.
On the plus side, the Birdy is a whole lot faster now that I have the tires sorted. I doubt that the Marathon Racer will have a significant impact over the Plus.