Zero Channel RAID (ZCR) performance problen

I have Supermicro X5DPR-TG2+ motherboard and Supermicro DAC-ZCRINT
zero channel raid controller.

I have strange performance problem with this combo:
Using onboard Si3112 controllers (SATA) I've got sequential transfer
rates on my wd740gd drives:
70Mb/s on single disk and 140Mb/s on RAID 0 configuration.
When I use ZCR in any configuration (single disk, raid 0, raid 1, raid
5) transfer rate is capped to ~55Mb/s. I use latest firmwares,
drivers. This cap is under Windows 2000, Windows 2003.
Who can help me ?