I was wondering about this and thought that, given in my case the core is skinned top and bottom and that grp is generally stronger in elongation than compression i would be better of if i skinned the upper side of the board in 450 carbon and the underside still with 600 grp.

My thinking is that i would have a better balance in the strength of the skins ( the upper being stronger to withstand compression the lower being slightly weaker but effectively strong in elongation) and hence a stiffer board than if i just skinned the bottom in carbon and left the top in grp...?

my understanding is that in a well balanced sandwich the top (ie the side in compression ) should be stronger  than the bottom as gf and cf are both stronger in tension. but i was just thinking i could make the top stronger by using cf.  i guess a well balanced sandwich is one that stresses around the center of its core hence being less likely to fail than one that is biased closer to one side or the other.