Reasonable expectations for Endo's first basho in j'oi really depend upon what type of rikishi one anticipates he'll be. As Krindel points out, the average of 5.38 wins for the first time in the top ranks means little, as that includes lots of rikishi with limited upper potential, and most of us don't view Endo in this light. For the most recent top ranked rikishi, their first basho in the sanyaku or j'oi went as follows:
All three of the most recent yokozuna, Asashoryu (K1), Hakuho (M3), and Harumafuji (a/k/a Ama)(M2) went 8-7.
For the most recent ozeki, Kaio (M1), Tochiazuma (K2), Kotooshu (M4), and Kotoshogiku (M3) went 9-6.
Chiyotaiki (M1) and Kisenosato (M1) went 8-7.
Three recent ozeki failed to kk on their first time in the top: Kakuryu (M2) 7-8; Baruto (M1) 5-10; and Chiyotaikai (M1) 4-11.
So, while admittedly this is a limited survey, 9 out of the last 12 ozeki got a kk the first time they faced the top of the banzuke. Therefore, I'd say the real expectations for Endo are 8 wins, not 6.