They're a few things going on here...
1. Dura Ace 8speed levers need to be paired with a Dura Ace 8sp rear deraileur. NO exceptions. The hub and cassette are NOT relevant.
2. Shimano cassette hubs are available in either low or high depth splining. An 8sp cassette should work on the 7900 hub as it appears to of the low spline variety.
740X, 7700, 7850, 7900 all have low profile splines and will accept an 8speed cassette.
7800 has high profile splines and will NOT accept an 8s cassette