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.
You may want to look at the North American Schwalbe Site
http://www.schwalbetires.com/bike_tires/road_tires
The Marathon Racer and the Kojak are each rated at 6 bars for speed whereas the Marathon Plus is rated at 3. (The rating system is: 6 is outstanding for class and 3 is average. I'm not sure what that translates to in rolling resistance)
If you open "Comments" on the Marathon Racer, The most recent comment is from someone who reports noticing a big difference when they changed from Plus to Racer on their recumbent.
The weights are:
Marathon Plus 510gm
Marathon Racer 260gm (250gm less)
Kojak 185gm (325gm less)
David