Itachi was able to keep up with KCM Naruto, guy won't be faster than that, and if he is Itachi whips out susano'o. No way is hirudora stronger than that.
Wrong. Naruto's Rasenshuriken is on the same
size as Itachi's Susano or almost... But my point is that the other techniques granted by the Mangekyō Sharingan, the user's eyes and body are strained while using Susanoo because it consumes a large amount of their chakra. Sasuke and even Itachi described it as feeling pain in every cell in their body which only grew from prolonged use.
AND one more thing... The only known methods of attacking the user while Susanoo is active are to draw the wielder out of the technique and then attack, bind and restrict its movements, or to break the defence with
overwhelming force like Guy's 8th Gate punch. Susanoo users are also vulnerable to attacks which don't actually deal physical damage to it, such as strong lights and sounds, which can also impair their ability to sustain Susanoo. Itachi's Susano can block every attacks except from behind. Tsunade has shown to break Madara's Susano, so by that Guy's speed and strength can actually pierce down the Susano. Look how powerful when Guy is on 7th and 8th Gate, EVEN Hashirama doesn't stand a chance.