First, if you haven't driven one, go do it. It's unbelievable how flat these cars handle from the factory. When I first drove a PP car, I was extremely impressed with how comfortable yet, poised the car was. Usually it's one or the other.
The GT's gained between 50-80lbs depending on which gearbox is in it.
The average price difference between a '14 and '15 GT (respectively) is about $1,000-$2000 depending on how they're equipped.
Most of the features available on a '15 401A weren't even available or produced in so few #'s, they might as well have not been available.
IMO, what you get on these new cars vs. the price difference (MSRP to MSRP) is beyond worth it.
GT's with an MSRP north of $43K have Adaptive Cruise, Recaros, the optional 20s, Reverse Park Assist, NAV, sometimes a 50 yrs. pkg, etc.
More stuff = more $$$