Well I can tell you this. I drove my friend's V8 firebird for a little while, and when I got back in my 8, while the acceleration feels less kick you in the ass type of acceleration, it definitely feels lighter on its feet. I can't explain it, but the car just "feels" faster than the firebird. The Firbird will definitely throw you in your seat, but once you start trying to throw the car around, it feels like a huge bucket of bolts, where the RX-8 feels like a feather.
I'd say the Mustang is probably a similar comparison. Think of it like the RX-8 is Bruce Lee, and the Mustang is Mike Tyson...