Do my metrics justify changing the MAXDOP settings? I recently took over a dual instance virtualized SQL Server with 8 x vCPUs and 8 x physical cores.  This is the only VM on the host.  

MAXDOP is at the default setting.  

The server has 100 high transaction vendor specific databases with some very nasty reporting queries.  

CXPacket waits are at the top of the waits list with 63%.  

Signal Waits are averaging 15.50%. 

In PerfMon the Processor Queue Length rarely breaches 1.  

Does this justify setting MAXDOP to 4 or 6?
I would investigate the CXPacket issues first - see what's causing those.  Can you identify a selection of queries that cause those wait types?  Can you then see what is causing the problem through the actual execution plan?  You're looking at gather streams steps, and looking for uneven loads coming in.  Try to identify the source of those - a likely suspect is out-of-date / not helpful index statistics.