Just wondering is the 03 STI standard Recirc good enough for around 400bhp? when is it needed to change to an aftermarket one?

 

I don't see why you would need to change it unless you actually ran in to a problem because of a malfunction with the O/E item. This assumes you are running the standard turbo and a modest boost increase.
The O/E item is pretty good. Until you flow much more air it should be OK.
I sold the O/E BOV from my WRX Wagon when someone needed it. I ran without a BOV for a short time but that was not a sensible option with the mods at that time and now it runs an O/E Seat Ibiza DV that cost nothing 420 bhp plus.