We know that the introduction of charm quark (GIM mechanism) suppresses Flavor Changing Neutral Current (FCNC) decays occuring via Electroweak box and penguin diagrams. But we have another doublet of quarks-top and bottom in the Standard Model. So, I am not able to understand how does the presence of top quark affect the FCNC decay? For instance b->sl+l- transition via Electroweak box diagram has 3 individual box diagrams corresponding to u, c, t quarks. If we consider u and c diagrams cancel, but what about top mediated box diagram..?
(with top mass being way higher than up and down quarks, the contribution from top mediated box should be considerable. So how is it GIM suppressed?)