I went through reliving my college physics and thermodynamics classes when writing the H,E,A.T. article here: http://www.instructables.com/id/LASER-HEAT/. But when all is said and done, a little bench work can can put those nasty equations into the closet 
Use a power-resistor and lab supply to heat the resistor to the same temperature as the Pi CPU when operating... Use an IR thermometer to measure the temp. Now, put the resistor on the heat sink oriented in the position it will operate and measure the temperature using the same power supply V & I. Note the stabilized temperature. Remove thee heatsink, reduce the I &E until the same temperature is reached w/o the heatsink as was recorded with the heatsink.
Your power supply values, I * E, gives the power in Watts between the no heatsink and heatsinked scenarios. Of course, our recordings are only approximations since mounting hardware, thermal grease, etc. Will have effects.
Ray