For those reasons, among others, Ford doesn't offer marketable reasons for Mustang enthusiasts who own 11-14 Mustangs to trade up, especially the Shelby owners.
Like yourself and others, I find the '15 mustang's performance to be rather disappointing because like any good car enthusiast, it's fun to argue and debate numbers. However, I find a lot wrong with the statement I pulled out of your previous post.
First off, the mustang GT is still a fast car. It's equally as fast (and maybe a touch faster, IIRC from the video in some areas) in a straight line than the 1LE. Secondly, for 90% of mustang purchases, the car will be driven exclusively on the street. Ford has made HUGE progress on the street manners of the new mustang. I know Lago didn't seem to notice much of a difference between this car and the previous one, but every other review I read of the car mentions how much nicer the new car is to drive on the street than the previous one. This is what will sell the new mustang to more people. And it's a very
marketable attribute, regardless of whether you're a long time mustang fan or not.
And then you talk about Shelby owners and that they wouldn't want to trade their cars in for a new mustang. Why would they? Who would trade in 500+ hp (depending on year) for a 435 hp V8? I wouldn't. And frankly, Ford isn't looking for Shelby owners to trade in their GT500 for a plebian GT. If you're thinking that this comparo bodes poorly for the next GT500 (or whatever Ford comes out with above the GT), I think you're also wrong. Although the GT wasn't as fast as the 1LE, Randy was gushing about how stiff the chassis is in the mustang. This suggests to me that a potent handler lurks within the s550 chassis, it just needs to get the correct tuning details worked out.
Let's not forget that when the current Camaro SS came out it was soundly trounced by the '11 GT, which was an updated version of an old chassis. What we're seeing now is classic ford. The first version of a new mustang is a little underwhleming. Over time, Ford slowly turns up the wick to something awesome.
People are rushing out to buy a '15 and that's great, especially for Ford. And heck, I'm sure the aftermarket will solve a lot of the handling issues we're reading about. But if you want a factory Mustang that can really handle, you're best bet is to wait 2-3 years to see what Ford does to keep the mustang fresh and competitive. There's no point in Ford releasing everything all at once and having nothing for them to improve upon--the car would get stale pretty quickly. Right now, Ford is riding the hype associated with the release of every new mustang.
Things will absolutely get a lot more serious once the dust settles from the release of a "new mustang".