Anandtech uses 340W source to measure the thermal conductance/resistance. But that does not mean it can cool 340W effectively. According to their result, D-15 has thermal resistance of 0.0944C/W, so 340W will put it 0.0944C/W x 340W = 32C above ambient temperature. That is 57C is your ambient is 25C.You are right there should be TIM between surfaces. Pressure is about 50-100psi. This is a very good guide on TEC mounting: http://www.customthermoelectric.com/TECmounting.html.