might I ask why compare a 2012 to a 2000?
porsche never cared about economy but now they have hybrids. they are pretty sad compared to whats out there but...
3.0L 8spd auto hybrid panamera S
2012 Porsche Panamera S Hybrid - Review | Hybrid Cars
thats about as good as you would get from porsche.
30mpg is conservative for the mustang. I believe someone here could easily be 45+mpg
also one other thing about the 2000 porsche boxer..
it would lead me to believe it is setup more for downforce and performance rather then low drag. cars like that can be a handful over 150mph with no downforce. or lift even.