10-11:1 compression ratio. A IHI turbo at around 38psi at 3000rpm, high quality fuel and very careful mapping!

The 300bhp comes early in the rev range and at the top of the rev range still has 300bhp. This is due to the boost levels, 38psi asap and 7-9psi at 7000rpm.

The restrictor is the problem, if you run too much boost the post restrictor depression gets too high and the turbo will, in short, die

Iirc down to the turbo overspending trying to suck through more air than possible