I got to jump in here since I own American, German and Japanese. When I had my 99 Boxster (pre supercharger) I ran against an LS1 Camaro (closed course) we were dead even til about 125 mph I started very very very slowly pulling away. Now we know an LSx based anything will destroy a Mustang. Tell your friend to go drive the thing and see if he likes it, He may be surprised.
This response was based on your question about 2.5 compared to Stang.
I'm very familiar with the 1/4 mile, I own an 11 second C5 Corvette.
Heres my time slip.
I'm not bashing your Mustang, just speaking from experience at the track. I love all cars by the way. The Mustang is one of the best looking American cars out there. Now the Challenger came out so my opinion has swayed. I think it is the best looking American car, period. I really want one, but again its a dog at the track.