But if you were to use a carbon  lay-up, the stiffness and
modulus of the carbon is different. If the carbon fails then you are already
above the load of the E glass

Dale,

 

I don't know what that means.  Our 320/360 main spars, while mainly made of prepreg, contain quite a bit of carbon in the spar cap.

 

One must always be careful not to change the local stiffness of a structure so as to not trandfer the load to some other unplanned location.