I have a gigabit router and gigabit switches throughout my house connected with CAT6, I have two 500GB Maxtor DiamondMax 10 7200RPM drives installed in RAID0 configuration. I see on average 20-25 MB/s for both read and write. FTP seems to be on the higher end of this average, and I sometimes see short bursts through FTP where the speed can reach 50 MB/s but that only lasts for a few seconds at most. I have noticed the more services I have enabled, (P2P, NFS, AFS, etc) the more it can affect the fluctuation of these speeds (always negatively). I try to limit the enabled services to FTP, and only enable the others when I need them, and then disable them as soon as I am done with whatever task I needed them for.
I have had no troubles with streaming HD content from this unit. For what I paid, I'm pretty happy with it. Definitely not the fastest NAS out there but its probably one of the best performers dollar for dollar.