Paul, Okay you asked for it! Here is my first article that seems to address prognostic ability! But first let me comment that some of these articles are rather old. Since this topic is new to me, I didn't mind reading the first studies done, but more recent research should have more accurate information!
http://onlinelibrary.wiley.com/doi/1.../ijc.10833/pdf
The next study seems to show that IL-6 is higher in women with high grade cancers and are more likely to metastasize
http://www.nature.com/onc/journal/v2...c2009180a.html
And, IL-6 is higher in women who have mets and the degree of mets correlates with higher IL-6 levels. Increased IL-6 levels indicates extent of disease
http://onlinelibrary.wiley.com/doi/1...ijc.10833/full
Ah, and IL-6 serum levels correlate to poor survival in patients with hormone-refractory metastatic breast cancer
http://www.nature.com/bjc/journal/v8.../6600956a.html
IL-6 may be produced by cancer cells and breast cancer cells that are sensitive to drug treatment do not express IL-6, whereas high levels of IL-6 are produced by multidrug-resistant breast cancer cells.
http://cancerres.aacrjournals.org/co.../24/8851.short
Also, IL-6 is inversely associated with KI-67 status and tumor grade (which seems to contradict early study on tumor grade)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2362311/
AND this one is just a great overview of all IL's and what is currently known!
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4307328/