Have you used the modified phatk kernel yet. Some have been showing it to have a 2%-3% increase. I rather suspect that eliminates the differences between this client and phoenix. mrb even acknowledged that the recent change was one that hdminer already had. Which since, like you I suspect that the difference is closer to 2% rather than 6% makes hdminer obsolete.
I get better hash rates with the poclbm kernel on the 6990. I tested a few of the optimizations people were trying out recently but those made no difference or made things worse. On the 5970, the modified phatk is much better.
However with the recent Ma changes and some other tweaks, I get better rates than the ones shown for hdminer. At 915, I get 817.6 compared to hdminer's 802. At 960, I get 857.8 compared to hdminer's 840. It doesn't necessarily mean that hdminer is slower since I don't have hdminer to try out but it does show that you can get better numbers than the published hdminer ones and anyone thinking of getting it should think twice about doing so. Perhaps there'd be another 2-3% increase if the tricks hdminer uses were used as well, as the Ma one was only about half of the 6% difference.
Upgrading to Catalyst 11.5 did increase hash rates a bit over 11.4. 11.7 is supposed to be even faster but it's still beta and I heard it was buggy.