There are many papers on PubMed for agmatine.
This one suggests it lowers blood pressure... (by relaxing blood vessels, like arginine does).

http://www.ncbi.nlm.nih.gov/pubmed/24800210

While this is a rat study (like most on PubMed)...suggestions of complex metabolic actions explain why body builder types seem
to use this more than others:
http://www.ncbi.nlm.nih.gov/pubmed/24523404

Keep in mind since most of the articles list NMDA antagonism, like magnesium and dextromethorphan, and the RX drug Namenda.... that this may be the major pain mechanism.
That also suggests you should AVOID MSG in foods because this additive is a factor in stimulating the NMDA receptor (pain receptors).

Wiki has a pretty good article on NMDA receptors for the list of agents that block it.

This article about reducing fibrosis is also interesting. Some people produce fibrosis in tissues, in abundance, and this can cause compressions. It is one mechanism in TOS pain, for example.
http://www.ncbi.nlm.nih.gov/pubmed/24505053
Another agent sometimes used in fibrosis is Trental (Pentoxifylline). There are some posts about this RX drug on our
TOS forum.

There seems to be a lack of human data on agmatine. One of the links in your post above involves only 5 patients. 5 is just not enough to be exciting at this time. Lots of rat studies though.