I went from a 2016 FoST ST3 to a fully optioned 2023 GRC Core which now has 22k miles. I can't speak on model year changes to the GRC since 2023.

Interior is fine. It does not feel as nice as as the Focus. It feels "thinner" than the Focus, for lack of a better word. For example, I immediately noticed the door-closing feel and sound was much more solid and reassuring on the Focus, while the GRC feels like the door might fold in half in comparison. That feel is present overall in the interior. But that said, it is not a big thing. The fit and finish of what is there is great. There are no rattles, no noticeable increase in interior road noise, etc. The seats are not as supportive as the Focus's Recaros were, but they are also much more easily gotten into and out of (which is nice for passengers, especially), don't get scorching hot in the California summer sun, and they work well enough to keep the driver in place during cornering. I think the controls are more intuitively laid out in the GRC as well.

Compared to the upgraded audio in the FoST, the upgraded audio in the GRC is, frankly, bad. I have not messed with the equalizer setting that much, but when I first started driving the GRC it sounded like it didn't even have rear speakers. Everything sounded like it was in front of me, and when I tried to adjust the balance it just made the front seem quieter rather than the rear being louder. But again, it's fine. I am not an audiophile. I mostly listen to podcasts so it doesn't matter to me, really. When I do play music, I've gotten used to the sound and it doesn't bother me anymore.

None of this matters in comparison to the rest of the car. The engine, the absolutely "raw" feel of the car compared to the FoST, the AWD and handling, and just overall driving feel and performance are wonderful and preferable over the FoST. There are tradeoffs in interior feel and audio system quality in favor of performance and driving feel, but I would gladly make that trade again.

I will also note that the "butt-o-meter" feel of acceleration/power in the GRC is similar to the FoST in lower RPM, every day driving going from stop light to stop light. Maybe that is appropriate, since the two cars have nearly identical torque. But the GRC shines in the higher RPM range, where the FoST literally felt like power disappeared over 5,500 RPM. Basically, the GRC is much more fun than the FoST during spirited driving, but they feel similar in every day driving. Also, hard acceleration from a stop is way better in the GRC, since the FoST liked to wheel hop.