Coefficient of thermal expansion and thermal conductivity are two different things. The first one deals with dimensional changes in the material with regard to temperature variations and the other one deals with how quickly a material transmits heat. Marginal leakage of a resin restoration, whether filled or unfilled, is more likely a result of the CTE, not thermal conductivity.
Unfilled resins distort with temperature changes because of its high CTE relative to that of the tooth structure's. This distortion occurs when a patients eats a hot pizza and then follows it with an ice cold drink, for example. The resin expands with heat and then contracts as it comes in contact with cold, while the tooth structure remains relatively stable. This phenomenon is known as percolation. As this is occurs repeatedly, the marginal integrity of a restoration collapses and you have a leak.
Anyway...who would fill a tooth with unfilled resin nowadays?
Ivorinedust
"Apolonia! You relieved my toothache!!!!"
