a true dual exhaust is bad for FI engines. why that term still lives on I'll never know. the closest you can get with out compromising performance is an offroad x pipe with straight pipe from there. You need to let the pressure equalize in these motors with either an H, X, or Y pipe. Yes you can still see gains from doing an exhaust but just like anything else its all about the weakest link. Change your muflers only, you hear it and thats about it. Change your cats, and exhaust, you might see some gain in track time, but to do an exhaust right, you need to swap everything from the heads out the back end of the car. And like doing an intake without and exhaust, you wont see max gains from your exhaust till you do other modification elsewhere. Your engine breathes...... the more you breath in the more you need to breath out. If you only breath in X you dont need to breath out X^4. Balance is key!
Think of exhaust like Weed.... "its a gateway drug." Yeah you get a minor high, but it only leads to freeing up the rest of the engines potential. And once you start with one, you have to do the rest. You may even justify it to yourself byu saying "I spent all this on an exhaust, i clearly need a kenny bell SC to make that investment in my exhaust worth the money."