His ERA might always outperform his xFIP, but that conversation is hundreds of innings away. And even if it does, it won't be more than a few tenths of a run. 4.77 xFIP Aaron Sanchez will need to become 4.3 xFIP Aaron Sanchez for any potential ERA-FIP skills to matter.
In other words, if he improves his peripherals by half a run, and has the ability to outperform his peripherals by a third of a run, he'll be a viable back end starter.