At least the 3DM2001 Nature and fillrates were scaling nicely from 990 to 1010 with the default BIOS (even though the timings are auto-adjusted at 1GHz). I got the best score so far at 1143MHz memory clock for that benchmark.
Could be different in 3DMark06 and 05 though - need to do more testing.

There is a fairly big drop in performance when you pass 833MHz Memclk. Memory timings are automatically adjusted @714, @833, @909 and @1000. The change @833 seems to give the biggest performance impact.

The test runs @950/980 have a custom VBIOS which keeps the memtimings at default ("714MHz timings"). It takes around 900MHz with these timings to match the default timings @980MHz.