SAS Expander decreasing RAID Performance?

[Teraman]

Hello,

what we have:

HP DL380 Gen9, p440ar with SAS Expander, 16x HP SAS 600GB drives, 2x HP SAS 400GB SSD

16x 600GB = RAID10

2x 400GB = RAID1 as SSD Cache for the RAID10

As you can see on the screenshot the performance of the RAID is realy slow. There should be atleast 800-1000MB/s with 16 drives in an RAID10, right? 

http://imgur.com/kjFGMiR 

The HP Support said it's because of the SAS Expander. I should remove 8 drives from the RAID10 to receive better performance, but this ist no option for us. 

All components are running on the latest Firmware. 

Best Regards,

Josh