I've hit 90-100 MB/s sustained without using SCSI -- using budget SATA drives. There's more than one way to skin a NAS...
"The problem" here is not really the hardware, it's more so the software. Any time you stripe more than one drive properly, with any reasonably modern drives, you're exceeding 120 MB/s maximum potential throughput. Put more than two of them together, and you should have really great potential, far exceeding anything you can do with standard GbE. Any decent GbE should be able to hit 900 Mb/s throughput. So if you put two such systems together with optimal software and configuration, and you should be able hit around 100 MB/s in theory.
Of course the hardware / software line is fuzzy at best, and if you had much much faster hardware then the software issues would be reduced. But my point is that the potential is already there with much of the hardware, it's just that the software and systems aren't so well tuned/designed for these levels.
The 100 MB/s was for example using ftp. I've never managed to hit that rate so far using native Windows file transfers (SMB 1.0) -- clearly in this case a software issue is holding back the performance.