I too am looking into the exact same thing. I have a '16 ST3 and have watched several youtube videos about exhaust for the ST. I've narrowed my choice down to either the COBB or MOUNTUNE exhaust, but I'd like some real-world opinions before I buy either.
I don't personally have aftermarket exhaust on my ST, but I know a few who do... I will tell you this.. the Cat-back setup from Ford Racing sucks, as well as the Borla exhaust setup... Both sound like BB's rattling around in a tin can, way too loud for a daily driver. People will just look at you and shake their head and think to themselves..."why?" I personally would go with Cobb or MBRP, nice mellow tone, while I have never heard either in person.. I have read enough online to know that it is a much more "balanced tone" than Borla or Ford Racing. But hey.. I'm 37 years old... what do I know?
Well I went ahead and bought the COBB system. Let me tell you what - this system is BEAUTIFUL and is SUPER high quality. The welds on this system are just astounding. The 4" tips just look bad to the bone. The tone is deep and mellow, and not too loud when you're driving around town. Get yourself out on a highway and hit the gas...that's a different story. It can get loud and angry. It won't make your ear drums bleed, but it's a little louder than I expected. I've been driving with this setup for about 300 miles now. I'm pretty used to it now. my buddies that have driven behind me say that it sounds awesome from the outside too. No "coffee can" sound here. Very impressed by COBB. Oh, btw...definitely buy the COBB polyurethane hangers. They're worth the $54.