A good friend has a 14 gt which I have spent a significant amount of time in. I love driving that car as it has the feel of an old school muscle car in terms of nose dive, body roll and how the rear just drops when you get it.
When I test drove my 15 gt with performance pkg it was a completely different experience. It definitely has not of a European feel and handling to it. Much better cornering and stability, minimal nose dive during hard braking and overall seems more nimble. The IRS is a huge improvement. While the additional weight will hole it back a bit compared to he 14 I am absolutely thrilled with how well it will outhandle it's predecessor.
It took a couple days for the rear exterior to grow on me but after a few weeks I would not trade it for anything.