Reply to "twin turbos"

Using any kind of tube headers has a negative affect on turbo performance, an exhaust manifold that has short runners from the exhaust ports to a common tube is best. The heat generated contributes to the force of the exhaust gas which is important for optimal turbo speed. The gases are still expanding after it leaves the exhaust port and pressurizes the exhaust manifold. Elaborate headers allow cooling of the gases and the larger volume (long tubes) to expand and have less pressure.
The advantage of using 2 turbos is that the size of the rotors will be smaller for the same boost as a single turbo, hence the mass will be less (maybe a squared function) minimizing the lag often associated with turbo applications. Controls allow sizing the turbo to reach the desired pressure at lower RPM�s and maintain constant pressure when the engine is at maximum RPM. Waste gates with intake pressure feedback is an example of such a device.
A properly converted engine, with o-rings in the head, oiling considerations and coil per plug ignition can double engine HP, unfortunately turbo sizing, plumping and controls are not well understood by many. Your limit of 5 to 7 lbs. of boost is conservative, it�s not worth applying two turbos for a 150 HP gain, a better route is traditional engine modifications.