When I ate there in November 2019, Chef Mizukami made me sushi. But, the sous-chef appeared just as experienced and capable - I wouldn't have cared the slightest if I sat with him instead. He also appeared more chatty than Mizukami san. The nigiri molding is such a small part of the overall experience. Ingredient/produce selection, preparation techniques of neta, type of shari and vinegar selection and strength, temperature control - all of these things are as important (or more) as the nigiri molding technique. In my opinion, people overestimate the importance of sitting with the head chef. Also, all the folks sitting with the sous-chef were Japanese when I visited.