D800 + SB 900 AF Assist On = Back Focus Issue

That has been my exact experience as well.

The SB-800 with AF assist light works rather flawlessly with the D800E, while both SB-900 and SB-700 produce massive amount of backfocus in the same test conditions.  I don't have the SB-910 to try; I won't be rushing to buy it anytime soon, either.

Interestingly, all three flashguns work just fine with the D700.