these things always give around 3 layers.
whether they are fast enough to do what you need to do - you will have to try and get a demo and test it.
it is not the benchmark data rate transfer rate that is important - it is the access times and rotational latency of the drives.
these things are dependant on the drives inside the raid, not the raid itself.

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We are currently testing xserve raids for speed, and getting at least 6 dv layers.
http://www.apple.com/xserve/raid/