I'm right there with you but I still don't think I can bring myself to do it. I grew up a Mustanger, and owned a series of them starting with the car I learned to drive on (my father's '70 Sportsroof) and my first car ('66 Emerglo straight 6 with 3-speed stick) all the way through '70 Calypso Coral Mach 1 show car, '85 GT, and '91 LX 5.0.
I have always lusted after '70 Boss 302, and when the recent one came out I considered it then. Everyone I asked then, including Mustang owners, said, no, don't sell an NSX to buy a Mustang.
I love the styling of the '15-16 Mustangs, and that 5.2 flat plane crank motor sounds delicious.
The only real reason I would consider the GT350 is that it's new, and I'd have it from new. While I enjoy having a 25-year-old car and having people go "THAT'S A 91?!?!?", it is not without its issues (see my post under electronics), although they are minor. I am on the fence, but I'd have to drive a new GT350 and be totally blown away to consider selling the NSX.
I've truly had the joneses for only three cars: aforementioned '70 Boss 302; early '70's Dino 246; and the NSX. So when it was feasible to actually have the NSX, I didn't look back, and it's been 10 years.
I'd have to drive a new GT350 before I'd commit. And like a dealer is going to let me do that. Um, probably, no.