A couple years back, I ran some calculations to determine what size pipes would be optimal fo a 4.0 re 244ci engine with 260hp.
Basically, my conclusion was the Xterras exhaust is for better words over built.
To be clear, I found our exhaust could support the flow needed of an engine with much more hp than our own.
Yes, there is a dent in the y pipe , but our engine didnt need a pipe of full size to flow appropriately to start with.
This can be seen in the post I recently shared.
AFE went out of their way to build the best exhaust possible.
They removed the y pipe dent, went up in exhaust pipe diameter, included a straight through muffler , mandrel bent their pipes and used stainless steel in its construction.
In the end, everything was optimized to deliver a properly balanced, first class exhaust focused to achieve a performance gain.
However, the best AFE claims is you may see up to a 5hp gain. I could gain this much power by replacing my air filter
If there had been a real issues regarding the dent in the y-pipe or how it effects exhaust flow, you would have noticed more of a difference in hp gains than what was advertised
Ps, I suspect the previous owner failed to maintain your vehicle properly which led to a misfire on bank 2 of your engine. This detail combined with it having higher mileage on the factory catalytic converters assisted with the emission failure. In so far as the owner having replaced both the front and rear catalytic converters, I would suspect this is the result of a good salesperson at the repair shop he visited