- Or would it be because of the GC50 thickness (but does not seem too thick for me) ?

The Biaxial is very cheap but adds great strength and stiffness plus once infused it is almost clear allowing the Carbon to show through giving the appearance that the part is solid Carbon Or if the part has no exposed edges you could replace the second layer of Carbon for Diolen. The second layer is only really there to prevent "transparency" between the nodes of the weave. 

Carbon Copies Ltd