The stock GT with $1500 in factory suspension kit erases any handling deficit the Mustang suffers against the M3. This $1500 is enough to make it the M3 equal/superior in all facets except style and interior material quality.
The Boss 302 is clearly an even better car. The best Mustang ever made, bar none and the choice of true enthusiasts who can't stomach the additional weight of the GT 500.
The F30 M3 with the turbo six has BMW playing catch up to Ford. BMW has a lot to prove to justify the excruciating premium they're asking, and the 2014.5 Mustang with IRS is going to really give performance-minded M3 shoppers reason to pause.