Will one of you guys who are running at 300 mem clock and worksize=256 give 225 mem clock and worksize=128 a try, and let me know your before and after results.
I don't have any 5830s, but all 58xx cards should behave similarly. On my 5870s I noticed that below ~250 mem clock worksize=128 works better. In fact, the optimal settings I've found we're at mem=186, worksize=128. That gives a few Mhash/s more than worksize=256 and mem anywhere between 250 and 400.