Really??
Am I the only one who thinks that Kenpachi takes this with mid diff at most whether he has gotten stronger or not?? He has been a beast for a long time now and his strength and spiritual pressure is far beyond the average captain. If he chooses to use both hands then this battle will be a walk in the park for him, but even if he doesn't I still see him taking it, with slightly more diff that is.
Mask is strong, that's for sure. But Kenpachi is simply on another level with or without Bankai.
Don't get confused with Kenpachi's personality and his actual strength before learning his training with Unohana.
Kenpachi is strong that's no doubt but a lot of people make the same mistake in thinking he's invincible due to how he acts and because of how biased they can get due to liking his character a lot.
Fact is Mask was able to beat 2 captain level Shinigami and a number of Lieutenants before being defeated by Renji is the most un charismatic fight ever. Kenpachi before his training would not have been able to do that purely due to how Kenpachi fights and his strength at said time.
Kenpachi before his training was always shown to be rivaling Byakuya take the fight with the espada the two had and how they were constantly ready to fight one another and I really doubt Pre Royal Realm Byakuya could have taken Mask.
I'm not necessarily saying your doing it however the reason most people say Kenpachi can low to mid diff anyone is due to how much they like his badass style and feeling-less attitude in battle but he does have limits and if he had battled Mask before his training he would have ended exactly like he did against fake Juhabach.