Unless the old and new car were tested side-by-side, it's tough to just look at the numbers and say one is better than the other. So many variables that can impact performance can vary day-to-day and location-to-location.
I'm surprised none of the publications seem to have tested a 2014 and a 2015 at the same time to get a better comparison.
The 2013 and 2014 are identical except for some paint codes.
2013 Mustang GT Premium Convertible
Candy Apple Red, Auto, FRPP Tune, GT500 Axlebacks, KONI Yellow Sport (adjustable dampers), FRPP Front and Rear swaybars, Custom Removable 4 Point Roll-Bar, BOSS 302 Low Expansion Brake Lines, Roush side splitters, GT500 spoiler, Redline Tuning hood struts,18" Ford Chromed Aluminum Wheels (9R3Z-1K007-A), Michelin Pilot Super Sports - 235X50X18
2011 Mustang V6 Premium Convertible - Gone