I have a brand new SCT BDX 40490 Tuner which can be used for all EcoBoost Mustangs up to 2021 (last time I checked). It still has the protective wrap on the screen. It is out of the box, I cannot seem to find the box. Again, brand new, never married to any car. Price: $300 shipped on me via UPS...
It should be 3 inch diameter tubing
You should be able to find all the specifics here:
https://www.mishimoto.com/ford-mustang-ecoboost-cat-back-exhaust-2015.html