Your Mustang's manifold absolute pressure sensor or MAP sensor is located behind the firewall and provides manifold vacuum readings to the PCM. Make sure yours is in perfect working order with a replacement sensor from Latemodel Restoration Supply.
LMR is a great company with great products. I recently purchased the Gibson Catback exhaust for my 2015 Mustang Ecoboost and I love it. Easy to install, great looking tips, and best of all almost half the price than competitors systems. My main reason I decided to go with Gibson was because off the reputation and because I think after searching all the exhaust systems, their exhaust system is probably the best sounding one. No raspyNess what so ever you expect from a 4 cylinder exhaust system. It's a nice low, deep tone.