I have max speeds allowed through a single gigabit port on my MBA.
I have flashed the BIOS to allow AHCI on all 6 ports including the external one as well as disabled the drive cache in the bios.
I have 4x barracuda green and 1 samsung HD154UI and a WD Green in there as well.
I'm using 2 x Seagate ST3000DM001 and 2 x WD Red. I've since tested the drive performance with a much bigger file size after reading this post and I'm getting about 150MB/s write and 300MB/s read. That's more like I expected.