P0471 Code on my Kia K5 after Exhaust Mod

This code appeared a day after installing a cat-back exhaust on my 2024 Kia K5 GT-Line. It seems to me that the pressure drop caused this problem, since the car has been feeling slower, and the fuel economy has dropped.

Now, I feel like I've diagnosed the problem, but I don't know exactly how to approach it. One of my friends suggested welding a flow plate into the midpipe to create some pressure. That sounds like the cheaper option; however, I've also seen on a lot of forums saying to tune, since I do also have a cold air intake and BOV installed. What should I do?

Or should I just put the stock exhaust back on?