Apparently Kubo has written a comment in the end of the novel regarding the huge power up Zaraki received, and according to him, it's all canon.

First part of the comment just thanks to the readers for buying the book and Narita for writing the novel. However it's starting to get ridiculous afterward, so buckle your seat belts.

According to Kubo, when he draws bleach, he leaves a few secrets in the manga that he expects onlya few readers to notice.

There are total 4 levels of secrets:

First level is the easiest, he expects every reader to be able to notice it.
The second level is a bit harder, only about 1 in 4 people to 1 in 10 people can notice it.
The third level is even harder, probably only 1 in a thousand can see though it.

The last level is something he put in from the beginning and something he only intends to reveal in the grand finale of the manga, things that he thinks nobody would be able to decipher, and one of those things was Zaraki's power.

When kubo read the ending part of the novel (the climax of the battle between Zaraki and that 8th Kenpachi), he said he was so shocked that Narita actually understood it and saw through Zaraki's true potential that he got so excited and happy that he read that part over and over again...

So, yeah, kubo was planning to reveal this at the end of the manga, but somehow this Narita guy spoiled us...

If you ask me, Kubo must have been on crack when designing this character, it's getting that insane here.

So to move on to the case, Zaraki's special ability (and also his handicap) is to automatically adjust his power level to his opponents, so he'll enjoy the fight longer (Makes perfect sense character personality-wise).

According to the novel, there were only two people Aizen didn't want to face before acquiring immortal body from the Hogyouku. One was Yama, so he created Wonderweis so specifically counter him, and the other one was Zaraki, so Aizen lured him to Hueco Mundo when kidnapping Orihime.

Acording to aizen, when Zaraki is fighting Shien in the novel (the Fraccion of Szayel, who was designed to replicate analyzed powers, and he mostly all the powers of the known Espada), Zaraki's strength at the moment is so strong that he can easily blow away the replicated forms of Baragan's Respia and Stark's wolves. Shien also used Szayel's voodoo dolls on Zaraki, he created dozens of them, but all of the parts of the dolls were too hard for him to break.

And finally, in the final battle between Zaraki and the 8th Kenpachi, none of the other spectators were able to sense their reiatsu, which means both Zaraki and 8th Kenpachi were effectively at transendent tier at the moment.

So to sum it up, zaraki is uber broken. When you fight him, he'll lower his power level if you are weaker, so he'll have a longer fight with you. If you are stronger, his powerlevel will slowly adjust to you. No matter how strong you are, in the end it will just come down to a swordfighting carnage of equal powers.