If you're running 1Gb/s Ethernet, you're never going to see the I/O that's theoretically possible with RAID5 even with drives running at 3Gb/s. The "network" (1Gb/s) is a hard bottleneck .
Thats why I have a 10 Gb/s NIC insalled and a direct connection to the 10Gb/s NIC in my PC - alongside my normal 1Gb/s connection to the NAS.
So It should be possible to see the maximum RAID5 perfomance - at least I hoped so. But turned out, I am stuck with 240 MB/s - even now that I have a 4 drive RAID 5 (was 3 drive before)