I am considering getting a NAS for home use (file hosting & media hosting (1080p) to ~ 3 devices simultaneously). I would like to set up 4 drives with RAID 0+1 and am wondering whether to buy a 4 bay NAS like the QNAP TS-419 P+ or the cheaper solution of buying a 2 bay NAS like the QNAP TS-219 P+ and add on a 4 bay expansion via eSata such as this. As the 2 bay + expansion is cheaper and you get 2 extra bays.
The eSATA extender's description says:
The RAID engine is embedded within the enclosure and performs all RAID parity calculation to reduce CPU utilization from the host QNAP NAS
So apparently this mode of operation that you want is fully supported. Now, I believe your network will be a bottleneck and not eSATA, so I don't think you'll get any performance degradation. This is of course assuming that the NAS box is sane and its eSATA port is as fast as its internal SATA ports, etc.
However, you're getting two boxes instead of one. Twice the amount of things to go wrong, etc. So it will be less reliable. But I think it will be just as fast for you.