This has been posted and discussed but it is vague.
BALLPARK FIGURES:
On the L28ET Stock longblock, turbo, and injection, depending on the exhaust system, you will see anywhere from 10-15HP for every PSI you add in boost up to a certain point. After 10psi total, you better be adding fuel and intercooling.
IF you start with an NA motor, you will make closer to the 15HP per PSI added BUT you will run into the detonation wall MUCH sooner. Guessing around 5psi max with the NA injection?
NOTE: The stock L28ET figures of 180HP are made at 7psi.