Generally speaking,comparing with FR4,CEM3 has better thermal conductivity performance,but no better than aluminum PCBs.
Now,Nanya from Taiwan provide high thermal conductivity cem-3 copper clad laminate,which makes possible to produce CEM3 PCBs with thermal conductivity like aluminum PCBs,the CCL type:CEM-3-09HT

Features:
Excellent in Thermal conductivity and anti-tracking property
The CTE of X and Y-axis before Tg under 20 ppm

Electrical property as well as chemical resistance are the same as those FR-4

Through-hole reliability and warpage have been improved in order to replace some portions of the FR-4 market.

IPC-4101C Specification is applicable

Applications:
.LED backlight module
.Outdoor lightig
.Automotive electronics