Both of the M8000 rear derailleur options indicate a max cog of 46, and the medium cage version only has a total capacity of 39. Usually those specs are a bit conservative, but you may have trouble getting a 51 cog to work well, especially if you have the medium cage rather than the long cage M8000 derailleur. If it doesn't work, you can swap in the M5100 Deore rear derailleur, which is designed for the 11-51 cassette.