Rick is spot on!

To make 300 hp on a cvh (with the standard poor flowing head) requires a huge level of boost pressure.
T3's and T34's are very inefficient at 30+ psi boost.
The YB will make 300HP at a much lower boost level.
It is not always about turbo flow - one must look at the turbo efficiency (or lack of) at high pressures.

Cheers