So if You're saying (as many other people have said) Itachi blocked Kirin with Yata mirror, That means Kirin did get though yata and susanoo. While if that was true it still blocked most of its damage but it still got through.
9 tails will do itachi....a huge chakra would be required to control a tailed beast like kyuubi....even if itachi has 9 tails in genjutsu it wont be long before he runs out of chakra n 9 tails is free n creating a huge tbb...
RIP itachi