Don't know why they didn't just name the 6990 into the 6890 instead, with 6870 and 6850 being single GPU Cayman cards, and 6770 and 6750 being Barts cards
Anyway, the 6970 and 6950 are kinda difficult to place yet - with Catalyst 10.12 being just released promising significant gains, and reviewers still mostly using 10.11 or a 10.10 hotfix driver