borla is a nice stainless steel exhaust system but is extremely expensive. Never heard Borla on a mustang before but I heard alot of people like it. If you dont mind dishing out the money for it then go for it
if you want some serious sound i know that SLP makes a "loudmouth" exhaust for the mustang. it is a true dual exhuast go you would need coustom dual or h or x pipe . whatever you do don't keep your stock y-pipe. it defeats the purpose of dual exhaust.
go check out www.mustangexhaust.com for sound clips. also, I'd avoid the straight flow through style mufflers on a v6 as those will be very raspy at high rpms generally. And you really don't need more then a y pipe on the v6 unless you go with a major power adder. The H pipe is only going to get you a couple horses on the V6 and removing your cats will probably make it sound like crap. That's my opinion though, I'd say go see what different setups sound like. make sure you have good speakers though!