A good place to go for more info on
Those power breathing turbos is John Deakin's series.
The way I understand it, the basic engine has a max power rating, limited by lots of structural and thermal factors inherent in the whole engine design. If you plonk a turbo or supercharger (or both) on it, all those other factors will remain (or even be exacerbated), so the max power rating does not change. Therefore, most systems will either have a bypass system to limit Sea level boost pressure or another system to limit overall power at sea level - to avoid the thing 'blowing up'. At the end, Take-off power at sea level therefore does not change.
However, what does change is that you can maintain that power throughout a climb and cruise - as long as your turbo/SC has enough puff, the engine will in essence continue to see sea level conditions, and therefore put out sea-level power levels.
Check out John's series - they are well worth a read. (And then come back and correct what I wrote - I*m sure I got something wrong : )
B.