Fuel efficiency tarmac sl8 vs sl9

I currently transport my Tarmac SL8 on a roof rack for my daily commute and driving to weekend group rides.

I’ve been reading up on the SL9 marketing and whitepapers, and it looks like Specialized made some pretty significant aerodynamic improvements to the frame and cockpit.

Since my highway commute is fairly long, the bike spends a lot of time on top of the car at 75–80 mph, where aerodynamic drag is a major factor.
I’m seriously considering upgrading to the SL9, mostly because the improved aero profile should technically reduce the car's drag and improve fuel economy. I figure over time, the gas savings will help offset a chunk of the cost of the new frame.

Has anyone actually calculated the aerodynamic drag diff of a bike mounted to a car roof?