Obviously the strength is supposed to be according to the numbers although the way the system of promoting through the ranks was explained, it's not necessarily so. Ie Mr. 5 and Mr. 4 pair are quite comparable in the technique they use (number uses bombs, partner strength, in a way) and I can see the 5 pair beating the 4 one on any other field than the one the pair 4 used in Alabasta. Quite frankly, though I really like him, Bon Clay doesn't have all that much and apart from being lucky in fighting Sanji (who couldn't kick Nami Swan for the life of it, he might beat Zoro if he clones Tashigi though :P ), he doesn't seem to be stronger than some of the numbers below him (after all he's working alone as well).