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...?