It's not the turbo that gives extra power. It's the air forced to the combustion chamber. So it's very simple more air + more fuel (to have decent air to fuel mixture) equals to more power. Actually connecting turbo to the exhaust decrease power of the engine, by restricting exhaust flow. Forced induction gives you more power that turbo is consuming so the overall power is higher than without turbo. Of course when turbo is spinning with the right speed. If not you have turbo lag when engine is weaker than similar naturally aspirated engine.
That's why you can't get straight answer to your question. There's too many variables in the equation. Turbo can easily double the engine's power or triple it, if the engine will handle such power. Most of the engines will handle 20-30% of power increase without other mods. If you plan more power you may need to rebuild your engine with new pistons, connecting rods, maybe even crankshaft. It's cheaper to swap more powerful engine than rebuild old one for more power. But it's a great fun to build one though
