[bucko]

Before you do anything, you need to get a new driveshafts. The driveshaft on the 13' v6 is very unreliable. You can get it here.
Mustang Mustang Aluminum One Piece Driveshaft (11-13 V6) M4611M - Free Shipping!

If you push it to hard without a new shaft, it could snap on you, which could destroy the entire car.

After a new shaft, I would proceed to get a tune, CAI, and then a better exhaust.

Good points if high speed and track use are an issue.

The stock driveshaft won't break at any normal speeds and use because of the stock speed limiter that's set in the Mustangs ECM (Fast enough for the posted speed limits in the states right now). A tuner will lift this, so If a tuner is to be bought and the plan is to lift the maximum speed for track purposes, then yes, consider the drive shaft.

My guess is that Ford changed the late V6 2013's with a one piece driveshaft for export sales. In Germany for example, they have the famous autoban sections of highway where the speed is higher, or even unlimited. For the Mustang V6 to sell there, a one piece would be a requirement.

 

[bucko]

The 2013's still use the two piece driveshaft

The late 2013's have the one piece. I believe this was due to exporting the Mustang to Europe, where they have certain sections of highway that have unlimited speed limits (i.e. Germany's autoban).