The way the catalytic converter efficiency is measured is by comparing the readings from the oxygen sensor in front of the converter to the readings from the oxygen sensor downstream of the converter. If both sensors are in good condition; this will be a fairly accurate measurement; but oxygen sensors sometimes go bad. When a sensor goes bad; it can create a false reading stating that the catalytic converter needs replacement. The only way to check whether or not the converter really needs replacement is to replace the oxygen sensor, clear the code with a code scanner, drive the car, and see if the code comes back. If the code comes back after a new oxygen sensor is installed; it probably means the converter is bad. But my understanding is that either the front or rear oxygen sensor can cause this code if it goes bad. So I would not just replace the front oxygen sensor; I would replace both oxygen sensors.
If the code comes back and still says that the converter needs replacement; I would not spend $3,000 on a converter from Nissan. The person at Nissan who told you not to get an aftermarket converter is either ignorant of the facts, or is lying to you. The law states that all replacement converters must be approved for installation on the vehicle model on which they are to be installed. Some converters which are sold by parts stores may not be approved for use on your car; but other converters sold by parts stores ARE approved for use on your car. The Nissan person's answer was one that would automatically prevent someone from buying the wrong converter; but anyone who checks and makes sure they buy a converter that is approved does not need to pay Nissan a $3,000 penalty just so they don't have to check what they are buying. The parts stores have the approval listings for the converters they sell. All you have to do is to look in their catalog, or ask the clerk whether that particular converter is approved for your car.
Cars which were originally sold in California require a different converter than cars which were originally sold in other states. The California approved converters cost a lot more at some stores; particularly if you get the exact fit models. But NAPA parts stores also sell a universal converter, which is available in a California approved model, for a very affordable price. This type of converter has to be welded in to the exhaust system. But a good muffler shop can easily do that. Here again; the Nissan guy doesn't want California people to know that a legal, approved converter is available for 1/10th of their price. But people who care about doing it right and also saving money don't need the Nissan guy to hold their hand and drain their wallet.