Your mechanic is a moron and I would not trust someone who told me that to change the oil in my car. With that said, you should get a new exhaust regardless just because an aftermarket exhaust is going to free up power over stock in most instances.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.