When you have a worn clutch the car loses power so when you replace in with a new one how much of an increase i power can you gain?

A clutch will not in and of itself give you any power. The clutch is simply a friction surface that transfers power from the engine to the rest of the drivetrain. If yours is dying (slipping), installing a new one that holds will allow all of the power from your engine to engage the drivetrain but will not make any more power. Upgraded clutches are for engines making more power than a stock clutch can hold. They have stronger clamping force and higher quality plates to hold under heavier load.