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.
The conclusion I came to with the 6870 was that the sweetspot was between 180 and 240. I was thinking the 5800s should be similar. There seemed to be a nice spike between those values, then below 180 the hash rates fell off rapidly. Did you find a similar pattern?