243 to the tires is huge. In addition to D/T loss, a turbo motor typically has a much "fatter" and broader torque curve than a N/A motor with the same peak power, and generally feels stronger throughout the rev range. This is especially important with anything but a close-ratio transmission.
Assuming the motor can stand it, more boost wont increase hp in direct proportion (50% more boost won't make 50% more power) but it may only require a bigger turbine and larger A/R. Also, surprise, increases gas mileage due to lower backpressure.
Why doesn't everyone just do this?
Because it increases boost lag (not response, but the time delay between when the throttle is opened to when you have positive manifold pressure), and raises the RPM at which boost begins.
You could have boost at all times (smaller A/R), but you'd need a larger bypass valve, and exhaust backpressure is higher which also reduces max boost.
