Well, my brand new WRX makes the "grindy deceleration noise" exactly the same as my old WRX made at 9,000 miles.
I don't think these gearbox problems get much better or worse.
Make sure you have an exact repro case, e.g. "drive hard in 1st, change to 2nd, change to 1st and decelerate for x seconds" to show it popping out.
The popping out of gear thing is worth getting fixed, but you need to find a dealer who knows what they're doing with transmission rebuilds. If you were in Portland Oregon I could tell you a dealer to go to who has fixed cars with this problem.