As far as I've been able to figure out, in the CaGEL* framework, complements are items that are licensed by some other element (generally the head), so that if an item has to be licensed, it is per definition a complement. But if I've got this right, then how come degree modifiers aren't complements? I mean, they elaborate on a feature that is only found with a subgroup of heads, just like any complement?
*Huddleston, R., and Pullum, G. K., 2002. The Cambridge Grammar of the English Language. Cambridge: Cambridge University Press.