That is interesting: I've done some testing on this type of card with 16 (slightly faster Hitachi) SSD attached. Setting WT and NORA should enable the so-called 'fastpath' mode for the card [1]. We saw performance improve markedly (300MB/s random write go to 1300MB/s).
This *might* be related to the fact that 16 SSD can put out more IOPS than the card can actually handle - whereas your 8 S3500 is probably the perfect number (e.g 8*11000 = 88000 which the card can handle ok).
I read that guide too, which is the reason why I tried with WT/NORA, but the document also states: "NOTE: RAID 10, RAID 50, and RAID 60 virtual disks cannot use FastPath." Which is a little odd, since usually if you want performance with reliability, you go RAID10.
Do you have any suggestions what I could try to tweak to get more performance?