Welcome to the support community & UCS!
In regards to your questions, unless the iSCSI SANs have dedicated ports just for UCS, or is solely to provide storage for UCS, then I wouldn't do direct attach. It's a far simpler design if you connect via your 5548's especially if you're going to server traffic to servers outside of the UCS system also.
Unless you have extremely high iSCSI utilization, I probably wouldn't suggest you'd need a separate uplink Pin Group either. Pin Groups can be used here, but they do add complexity and having a multi-10G uplink port channel you'll likely never push it to capacity anyway. A single uplink port channel should be adequate and keep it simple at the same time.
Along with this allow the system to round robin your vnics to uplinks. If you're not using Pin Group Uplinks, then you don't need to do vNIC pinning. Again, more complexity and in your case unless you're versed on troubleshooting and supporting it, I would stick to the straightforward design.
What I would suggest is the following:
- Put all your iSCSI traffic on a separate L2 VLAN.
- Try to locate your target iSCSI SAN on this same VLAN (avoid routing and packet fragmentation)
- Enable Jumbo frames on this VLAN End-to-End. (There are multiple whitepapers and posts on this topic). iSCSI + Jumbo Frames @ 10G is the closest thing you'll get to Fibre Channel performance on a SAN.
- Setup one of the UCS Class of Service queus for Jumbo frames (this will set your MTU accordingly). One thing that's often missed here, is appropriately marking the CoS on the return traffic. You would do this on the interface & switch the iSCSI SAN directly connects to, which should be the 5548's in your case.
If you have any other questions, let us know.