By disabling TCP Offloading your telling the TOE NIC(s) you've dedicated to the iSCSI network to use the CPU to encapsulate and de-encapsulate the iSCSI packets. This may not be a big deal if your CPU utilization is very low and your packet transmit rate is moderate or less. If I remember correctly high iSCSI packet rates would increase CPU utilization by as much as 20%. If don't have any technical reasons to turn it off I would leave it enabled.