20 psi is 450 hp if everything else in the engine has been grabbed by the neck and choked: ported head with lumps, big cam, high ratio rockers, big valves, tuned exhaust.
Power obtained from boost is never linear, the first 5 psi is far more valuable than going from 15 to 20, the results decay as boost goes up.
In round numbers (assuming the original engine to give the factory rated 155 hp, atmospheric pressure is 14.7, and the WOT vacuum is 1.5 psi), 20 psi with no other changes will make about 45% more than stock or 225 hp.
670 hp is a $10,000 engine.
The M90 produced 260 hp in the original 231" V6, so it's going to run out of capacity with a bigger engine and higher boost.
