So, show me a scan that says Thor's force is magically multiplied through some sort of mystical enhancement.
Because - yeah, you can hit harder with a hammer than with your fist because of the increased torque, and the decreased surface area. But besides the purely MECHANICAL advantages a hammer bestows upon Thor, it's never been said that the hammer changes his force in anyway.
Give him an adamantium hammer, and he'd replicate any 'supra-destructor-bolity' feat you want to name.