I'm thinking though if an AQ6 is put there to control 15 fans, won't it become a giant clutter of cables?
No... In my build I simply ran 2 cables from the AQ6 and these two then connected up to some 'Mod My Toys' fan splitter boards [I use 2 fan headers on the AQ6XT to control 17 fans - 8 rad and 9 case fans]. The fan splitters can be mounted way out of sight and in a convenient location near the fans. Then you only need to run one cable from those boards as the splitters effectively become a commoning point for the fan cables.