I've noticed the same thing as you.
I have a 2011 GT and was thinking I was going to get a 2015 or 2016 GT with the improved 5.0. Now I'm pretty certain the GT500 is the way to go.
I had a 2008 Corvette Z06 and was trying to decide if I should buy a decked out 2014 C7 Vette or keep the Z06. Well same thing has happened to ZR-1's that happened to the Shelby, the bottom fell out. I replaced the Z06 with the ZR-1 and it was definitely the right choice. I was able to find a decked out ZR-1 in the right color for $30,000 less than new.
There is nothing like having the top dog of the pack and the ZR-1 and the GT500 are the top dogs until the new ZR-1 and GT500 arrive. I don't think the replacement for the GT500 is going to have as much horsepower. That's the number that people strive for as a collector car and that's the number that gets a Wow factor.
I think additionally the GT500 is such a good driving car that even though the 2015 is going to have the IRS, I think the GT500 will handle better and run a road course faster. I'm certain the GT500 will rule stoplight to stoplight and the drag strip.
Lastly, I don't think there will be much depreciation past where they are now with them as long as the miles are right. So if you decide a year from now you made the wrong choice, you eat the $2-3,000 in depreciation and go for the new 2015 or 16 GT.
EDIT: It's late and I didn't engage the brain completely. I'm thinking 2013 GT500 is what I'm going to do. But now that you have brought the low prices on the 2011-2012 cars to my attention, I'm not sure it's worth the extra 10-15,000 for the 2013.