Hi
Here is my thoughts
Even though each NIC dedicated to iSCSI traffic will now be 10Gbps, vs 1Gbps, will reducing the physical NIC count to (2) serve to defeat the purpose of the NIC upgrade?
No, it will actually be better, you will have more bandwidth, less cabling, simpler configuration.
What ramifications are there to no longer having the iSCSI traffic destined for the MD3800i separate from that destined for the MD3220i?
It depends on your workload, but this should not really matter at 10GBe, also MD3220i cannot do more than 1gbe anyway.
What are the ramifications of defining (4) paths on (2) vNICS for traffic destined to the MD3220i (one path for each subnet defined on the MD3220, as best practices dictate)?
Here is the best practise running iscsi storage with vmware http://www.vmware.com/files/pdf/iSCSI_design_deploy.pdf - Also, you want to avoid multiple subnets for iscsi, here is a good read http://wahlnetwork.com/2015/03/09/when-to-use-multiple-subnet-iscsi-network-design/
Since the MD3220i only has 1Gbps ports - (4) on each of (2) controllers, would it make sense to reconfigure the iSCSI ports on the MD3220? Currently there are (4) subnets, pairing the corresponding ports from each controller (I.E. 192.168.230.x = ports 0/0+1/0, 192.168.231.x =0/1+1/1, 192.168.232.x = 0/2+1/2, etc.) into (2).
As above be careful with multiple subnet for iscsi, please read Chris Wahl article. Will it make sense for iscsi ports??? depends what other protocols are available for this storage, NFS, FC and so on.
The S4820T switch is auto-sensing, so we are being assured we can attach the (8) ports from the MD3220i, but what are the ramifications of a 10Gbps NIC attempting to negotiate with one or more 1Gbps port? I have read stories of the 10Gbps NIC falling back to 1Gbps entirely until communication with the 1Gbps target is complete. This would be highly undesirable. Has anyone encounter such an issue?
It will work, backward compatible, but you will not be able to get more than 1GBps of speed.
Finally, has anyone made a similar move with similar hardware?
It is not complicated, you just need to plan it carefully, and do it in maintenance window. If you have free ports on the storage, it will help as you can use that for test initial configure is working for your new switches.