I question if it actually needs the canister. You also fail to mention the code and there are a couple of technical service bulletins released by Nissan for specific evaporative emission codes. One is that there is an updated evap canister check valve, which has a stronger actuator. The other pertains to a filter kit that has been created to prevent dirt from entering the check valve. The only reason a canister would need to be replaced on an R51 is if it is cracked or damaged (hard to get damaged where it's at on the vehicle) or if the internal foam liner has broken and is letting charcoal bits into the system; this was more common on early R50 Pathfinders than it is on R51s. You might want to question the reason for them replacing the canister as opposed to installing the updated check valve and, if needed, the filter kit.
Generally speaking, the evaporative emission system will usually not affect performance. It is a system designed to store fuel vapors from the gas tank until a later time when they can be sucked into the intake and burned through the combustion cycle of the engine, rather than have those vapors exit into the atmosphere. The system employs a self-check system to verify that it is working properly, and if it is not, will trigger one or more trouble codes, depending on the failure detected.
The check valve is available on the aftermarket. The last time I checked, Standard Motor Products had the updated valve while Dorman was still selling the original style valve, so spend a couple of extra dollars and get the updated style valve. The difference is fairly obvious when you compare the two. The filter kit is available only from Nissan.