Alex, oem gearbox internals, namely the centre diff is the weakest link. Once you've upgraded that (4 spider or welded/spool centre) the 4th gear is the next weakest link. Some people get away with not breaking the centre diff or stripping 4th gear with 400hp atw though, but they are probably mindful of how they treat the driveline.
Once you make more power than 400hp atw you've got to start seriously thinking about that centre diff, 4th gear and twin plate clutch upgrade as you WILL break stuff.
Transfer cases have been known to blow anywhere between the 400-500whp figure so it makes sense that when chasing that magical 500whp that the transfer case and driveshaft yoke/U-joint get attention too.
After that you could then be breaking the rear axles.
That's probably why 300kw/400hp atw is what most guys are content with in a street car with an oem driveline.
It gets very expensive really quick when you're making more power than that.
It's too much power for the street, but its a lot of fun finding out I guess