Not true quite a few type-s have broken 300whp with a greddy kit and 3"downpipe/exhaust.Greddy will never make that kinda of numbers. The turbo is way too small and you would be lucky if you make 270whp out of the kit.
Ts, if you know you will be content with 300whp and never want more power then greddy is the kit for you. As far as internals you dont need any for 300whp.
Here's a complete kit that you can consider with 3" down and exhaust