Sunrace Wide Ratio 9-Speed Cassette Questions

[Kerchak]

Reviving an old discussion.

Busy converting my old 3x9 to a 1x9. I have a XT M772 derailleur. Looks like it is a long cage. The distance between the pulleys is approximately 10cm.  From the discussion above it seems like it will work with the Sunrace cassette.  My LBS said it is not going to work and that there is actually another derailleur from Shimano specifically for the wide range cassette. 

Any feedback/advice on the XT derailleur and the Sunrace wide cassette would be appreciated.

PS.  Have been to several shops in PTA east and nobody has the 9sp wide cassette in stock. Everybody waiting for stock