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?
How did you get your mem down that low on the 6870? Did you flash the bios?
MSI Afterburner.
Enable unofficial overclocking by editing the config file. Afterburner will appear to only let you take the memory so low, around 525 Mhz at first. Close it down then reopen it. Now you can take it down even lower, to around 260 MHz. Close it down, reopen it, set to 225 Mhz.