The 2013 GT500s just dont make enough power for some of you guys. We have confirmed Cold air intake fitment from JLT on the 13s, we have also confirmed the Steeda 10-12 intake does not work on the 13. Alot of sites have this incorrect on their webpages right now. Metco is releasing a 2.4 inch pulley within the next week (We may have weaseled one out of them ahead of time). Stage 3 is able to custom tune the 2013 GT500s right now as well.
Here are some popular options.
2010-2013 Mustang GT500 JLT "Big Air" Intake (Plastic)
2013 Shelby GT500 Metco 2.40" Supercharger Pulley
2013 Mustang GT500 SCT X-Cal 3 Tuner w/ Custom Tune
2013 Shelby GT500 5.8L Stainless Works Cat-back Exhaust System (To Factory Headers/Mid Pipe)
The stainless Works exhaust systems sounds amazing on these, take a look/listen below.