860-940 here, 6 5850's.
900 isn't a sucky card, it's average.
At least for me, 5850's run at a lower voltage (1088mV) than 5870's (1163mV), so it's not surprising that they don't clock as high really.
Seems like my card is normal then.
5870 run at a higher voltage because they have more SIMDs, TMUs and shaders.