I'm telling you, all that pain he's taking in is being saved up for a MASSIVE Ursus Shock attack. He's had YEARS of torture before and after he became a cyborg. He's endured so much and IMO we're going to see a Kuma with his mind back in this arc.
Theory time: Honestly, based on the flash back with Kuma talking to Saturn, I think Kuma is going to unleash this charged up Ursus Shock on Saturn... Defeating him, the first of the Gorosei with the help of Luffy. Kuma will die soon after and EVERYONE is going to be freaked out. I think the other gorosei are going to take action immediately, which is going to cause a crazy HUGE war with literally EVERYONE involved.