What was the limiting factor for me was cooling. With a big external fan blowing on the cards (2x6990), I was able to do 905 MHz and 625MHz underclocked memory (using MSI Afterburner, couldn't get it down unfortunately to ~300MHz), and it was running somewhat stable.
On my other box (TJ07 case with 2x 6990s running 11.04) I had to drop my clock speeds to 830MHz with 294MHz underclocked ram due to case cooling issues. I'm looking at consolidating all my 6990s in one box and using some proper water cooling to keep things running cool. At that point 925+ should be easily attainable.
The thing is you don't want the card's temperature nannies to kick in. If they do, you suffer a bigger performance hit than running at a lower speed. I think it's safe to run the cores up to 96C.
What I also find interesting is that ATI's default setting allows my cards to heat up to about 100C. Wonder why they did that?
So your thoughts on default cooling, 450W and 900Mhz?