It took very little to get my 2011 to handle really well. Just shocks, struts and springs eliminated all the squishy handling. I'd expect the IRS to make itself known mid corner over a bump at 8/10ths.
That's what I said before I drove a 15.
But the suspension improvement was very noticeable. As the OP notes, nose dive during braking is much reduced, and the front end doesn't raise up nearly as much under acceleration.
So the power is the same, but the driving experience is definitely different. I would argue that it's improved. Obviously because it was a dealer car I didn't really push its performance potential, but it was a noticeable difference.
That said, would I buy a 15 GT? If I didn't love my 14 GT as much as I do, then maybe.
2011 GT | MT82 | 3.73 | Brembos | Barton | Airaid | CDC chin | Corsa X pipe & Xtremes | DSS Aluminum Shaft | BMR Rear loop & LCA relocating brackets | J&M Extreme LCA | Koni Yellows | Steeda Sports | UMI Panhard | SCT & BAMA
"Yo, nutsack! Let me tell you something, man, muscle beats import every time. You know what I'm saying? Every time!" - Dwight