Computurd,

8 drives is double the amount of drives he's got to play with. (it doesn't matter if he's got 6Gb/s devices)

As I commented, if you look at the ATTO benchmark, the throughput is there, it's just that is located around 128KB-256KB and not where the typical benchmarks programs are measuring the throughput for sequential io's.
It should be easily reproducible using HD Tune using 128KB block size or iometer using the same block size.