I bet the rear diff or the exhaust just hit something... or each over... they both move around a lot.
You probably haven't done any detectable damage to the gearbox... just chill out... if it break it breaks, worry about it then.
Been driving it today and I notice that when I shift into third from 2nd or 4th it is making a kind of noise. It is hard to explain and it is not very loud but it sounds ling a "ching," noise, like 2 bits of metal clanging together very quietly. The car has always done that when changing into 5th gear but never in any other gear and it is usually quite quiet, however it is now doing it in 3rd gear which is the gear i accidently slammed and it is quite a lot louder. It is still not easy to hear exactly, but it is making me anxious.
So I can prepare, I am getting my car tuned soon to run about 250whp which I know will put more strain on trans. I have done some research and it said around 300bhp or 250whp was the upper safe limit for stock WRX 5 speeds. If the gearbox does go, what should I replace with to keep cost around $1,500?
I don't really understand the options as it seems like you can rebuild, recondition, replace etc.
Do 5 speeds from say a Hawkeye WRX or a 2012 WRX fit in? Are they better to put in? Is there another option that would be a slight upgrade at not too great a cost?
Shock load is generally 'at launch' a crunchy change is less of a load issue as you already have forward momentum, in your example 'it still works' and that is a good sign
Some of those little crunchy bits may not be attached anymore though, and sloshing around in the gearbox lubricant, draining said lubricant at the next service will tell a tale, think of it as reading the entrails, but no animals are harmed in the process 🤣