It isn't boost dependent. Generally it should go by the size of the turbo. If you are running something like a 20G or larger, the 7 series plug should be used. Even then, you can use the stock plugs and be fine.
A Big16G can run 20+psi no problem, but you can still use the stock plugs. The 7 series plugs just helps keep things a little cooler. The engine is designed for use with a 16G anyway, so stock plugs are great.
