I put an OEM one on my 14 GT soon after I bought it. I really didn't notice any difference after the install but then again, I didn't drive on any winding roads before the install so maybe that is why.
If you want a factory STB, I bought mine off of Ebay from a place that had new takeoffs. I think mine was $99 shipped. Sometimes people on the different Mustang forums will sell one. I like how they look so I'm glad I installed one.
2014 GT, Sterling Gray, base model, Automatic
GT500 AB, GT500 OTA pipes
Bob's oil catch can
Ford aluminum finned diff. cover
Ford all weather floor mats
Ford trunk liner and cargo net
Ford locking gas filler plug and wheel locks