I'm quite interested to see what the solution is here as well. I'm having the exact same problem with a Corsair Force 3 SSD. I've got the the SSD (as a single disk) and two HDDs in a RAID 0 array on the AMD southbridge with SATA 3.0 enabled in the BIOS and the best read/write speeds I'm getting in the ATTO benchmark are around 240 MB/s, less than half what they should be. It's showing up the same way too, it will hit a wall right around the 64k mark.
Between the Etron USB 3.0 issues (which resulted in me having to buy a new external enclosure for my HDD as I hear enclosures using the Asmedia chip, rather than the JMicron, should work without issue on the Etron controller) and now this SSD headache I keep expecting this motherboard to climb out of the case and kick me in the nuts just to prove how much it hates me...
EDIT: I think I found it. Do you have RAID 5 support disabled in your BIOS? I did, since I had no plans to ever use RAID 5, and found a thread on here talking about that impacting drive speeds. So I went in to my BIOS, enabled RAID 5 and voila! ATTO benchmark speeds in the 400MB/s range. This is a really odd quirk but I'll take it.