FYI Sig has a flat takedown lever available for the P320.
My thumb looked the same way after every range visit, but my original takedown lever would get HOT after only a mag or two. I wasn't even rapid firing. Sent it to Sig and they replaced the lever, no more heat issues.
GrayGuns just released trigger kits for the P320. Almost makes me wish I still had my P320. Maybe I'll buy another one down the road.