Sometime around '08 +, (not exactly sure when)Toyota changed the catback tubing from 2.25" to 2.5".
The 2012 definitely has a 2.5". So to answer your question SoCalTacos, No. You OEM catback will not be a huge limiting factor if you add in the URD Y-pipe.
If you were going to be boosted, then I would say a 3" catback and the URD Y-pipe mod.
An example of how the URD Y performs, with headers, intake, & OEM catback can be seen in the link for Lucario Runner's 4Runner:
His power went up by 26whp and 30wtq by the end of the run. Just by adding the Y-pipe. It made a significsignificant difference, and still sounds 'tame' with his OEM catback. Which also has a small resonator.