Alright.. I'm confused.
Why would a wider tire be more efficient? More weight and certainly more wind resistant. What's the Giro? I could understand maybe going to a wider tire is the roads are broken/crappy and you want more rubber on the road. Am I missing something?
As to rolling resistance, there's an optimal amount of tire deflection for minimum rolling resistence. Thus depending on rider weight and tire pressure, wider can have lower rolling resistence.
As to aerodynamics, you're correct that generally wider = higher wind resistence. However, there is also the issue of how the tire interfaces with the rim. With the new generation of very wide deepsectioned rims, like Zipp Firecrests, racers are finding that there is little or no aerodynamic penalty to run a 25mm tire on the wider rims.
My point above, though is that you can't just extrapolate out, based on team weight alone, because the aerodynamics of a 28mm tire on a typical tandem rim, are going to be different than a 25mm tubular on a Zipp firecrest rim.
Zipp firecrest rims have a maximum width of 27mm. Thus, you can see that a 25mm tire might not add drag, compared to a 23mm tire.
By contrast, a Rolf tandem rim is narrower than 23mm, so going to 28mm is likely to add drag.
All that said, you can't make a definite conclusion on any wheel/tire combo, without a wind tunnel test, because what's intuitive, is not always true in aerodynamics.