You are correct stock injectors will not go past 270 whp, well from my experience. I have tuned to many cars and noone makes it past that, though all stock fuel pressure and stock fuel pumps.
Also the stock fuel pump is a big problem at around 300 whp, as it can nto push anymore fuel and runs out. So you will also need to upgrade the fuel pump to at least 255lph if not bigger.
Another thing the fuel lines that run from the gas tank to the engine, are not big enough to run fuel through at around 350whp, so you will need to upgrade the fuel lines also all the way from the gas tank.
As for 350-450whp 650cc injectors will cover you for that amount, you can always raise the fuel pressure a little more, but that shoudl work fine. Just make sure it is well tuned and you have a big fuel pump, like 315lph.