It is great to see a larger dedicated Grand Seiko presence, and as it is another floor and part of the main Seiko museum it is an easy must see for visitors to Tokyo. When you go please be sure to also check out the Seiko levels of the museum to get a better understanding of the history of the brand and how Grand Seiko fits into this.
When visiting I would suggest making a free reservation though the official website. This is not essential but due to the limited spaces available it is strongly suggested for international guests to ensure you get access at the time you wish to visit.