I apology again if I went too far - i looked back and a few things can be better said;
I have not, but I can; look at what iscsi is doing on my ESXi DSM(5.2), and see what is required for good block access.
my setup will NOT be a good test case, I am using SHR - which in all aspects, is file level (or more correctly - quite a few layers to get to block level) - so iscsi will be hampered by that.
if DSM is using decent raid (raid-5 with write hole, ) then you should be able to get pretty decent speed.
looking at DSM's site :
https://www.synology.com/en-us/knowledgebase/DSM/help/DSM/StorageManager/iscsilun
looks like it explains it better than I did;
but then as you said "devices specifically created for iSCSI, can take advantage of iSCSI and provide the actual performance that it was supposed to offer, everything else feels like a poor-man's version of iSCSI"
we can do some tweaking, and some MPIO work - LUN packet sizes, proper raid; LUN allocation unit size (should match your type) but if you really want/need iscsi - DSM may not be for you.
for 99.999% of home labs, early mount NFS via fstabs (or other node supported ways) is going to get you what you need.
I do not mean to Lecture; there are things I learn every day myself.