first of all...i think if you copy from RAID6 to RAID 10 it is fine and you are getting almost what you expects
and from RAID10 to 6, you are not getting...
I think the answer is simple... RAID10 is not for heavy read but good heavy write and RAID6 is good for heavy READ and not for heavy write..
then another important one, you are using ISCSI, and the 4 GbE throughtput which you are expecting you wont get, because in the switch this speed is the BACKPLANE speed, and there is another parameter in the switch called MPPS million packet per second or packet forwarding rate etc in the dell case
Switch Fabric Capacity 136 Gb/s
Forwarding Rate 95 Mpps - this is the main parameter for the switch performance.... and this rate is with the MTU of 1500 or less and if you enabled JUMBO frames then the switch should have more forwarding rate, so with 2 switch you get nearly 200 mpps with MTU of 1500, and with MTU of 9000 it needs more power in the switch.
these all comes in to picture when you design the ISCSI/NFS