Also optimistic to have Redbulls 7th Gear to go up to 370, whereas they already are using 8th gear in Bahrain at about 310km/h.
I had a look at the Merc pole lap posted in the Bahrain GP thread and Rosberg in 7th is at 325kph at 11500rpm which agrees with the graph. The Red Bull at 310kph is around the speed you want to change to 8th gear as indicated by the graph and concurs with what you say about the Red Bull changing to 8th at 310kph, so there's nothing to indicate that the graph is wrong.
Maybe Ferrari feel that a higher top speed is not worth it considering the amount of circuits you could potentially use that top speed is quite small and dependant on when you can use DRS. Although the graph indicates that Sauber and Marussia have a similar gearing to maybe it's a consequence of having a Ferrari engine.