The DC kit is a nice kit, all T-304 3" 16 gauge throughout. Excellent quality.

The thing to remember is the system is 3", but you've got 4" double walled tips. The piping is a full inch smaller than the tips out back, so it's not nearly as big. You can use a 3" system just fine on a factory setup and not mess up your back pressure. You were smart and did what I recommend: a full catback. The guys having issues on these cars are the ones who are straight piping them or mixing/matching parts. The catback systems are specifically tuned for your exact car and tuned for the stock tuning parameters. You do not need to worry at all about the exhaust being too large. You also don't need to worry about damaging the motor because you don't have straight pipes. The catback has a set of two specially designed mufflers that still regulate your exhaust flow. Yes, it is increased, of course. But it is still within spec to work without a tune on an otherwise stock car.

I say enjoy it and don't overthink it. You're fine running that setup.