I actually did find a study about loss of productivity due to chronic pain. It doesn't relate or compare the loss of productivity due to untreated or undertreated pain, but given our own experiences it's a good guess that most of these people with chronic pain were being undertreated.

Here's the link:
http://www.partnersagainstpain.com/html/profed/pmc/pe_pmc3.htm?pg=7631