Yes they can easily see 30 ft and move that far in one flip of the tail. There is a balance between too much structure and not enough for fishing purposes. Too many and the fish will be scattered. You want enough to do the job (achieve the purpose - place to feed , hide and move around) but not so many that the fish are not concentrated.

IMO the top of the structure for deep water should be at least 4 feet wide. You don't want most all of the structure bulk in one depth zone.