Connecting controller to any of the three PCIe x16 slots yield choppy read performance around 750 MB/sec

Lowly PCIe x4 slot yields steady 1.2 GB/sec read

Given same files, same Windows Server 2008 R2 OS, same RAID6 24-disk Seagate ES.2 3TB array on LSI 9286-8e, same Dell R7610 Precision Workstation with A03 BIOS, same W5000 graphics card (no other cards), same settings etc. I see super-low CPU utilization in both cases.

SiSoft Sandra reports x8 at 5GT/sec in x16 slot, and x4 at 5GT/sec in x4 slot, as expected.

I'd like to be able to rely on the sheer speed of x16 slots.

What gives? What can I try? Any ideas? Please assist

Cross-posted from http://en.community.dell.com/support-forums/desktop/f/3514/t/19526990.aspx

Follow-up information

We did some more performance testing with reading from 8 SSDs, connected directly (without an expander chip). This means that both SAS cables were utilized. We saw nearly double performance, but it varied from run to run: {2.0, 1.8, 1.6, and 1.4 GB/sec were observed, then performance jumped back up to 2.0}.

The SSD RAID0 tests were conducted in a x16 PCIe slot, all other variables kept the same. It seems to me that we were getting double the performance of HDD-based RAID6 array.

Just for reference: maximum possible read burst speed over single channel of SAS 6Gb/sec is 570 MB/sec due to 8b/10b encoding and protocol limitations (SAS cable provides four such channels).