SQL 2012 :: Determining MAXDOP And Memory Configuration For Multiple Instances?

We are just finishing our migration to SQL 2012. In our old environment, the instance which held our SharePoint databases also served other applications. We did not experience any performance related issues in the past due to this.

SharePoint basically requires MAXDOP to be 1, which is correct on the old server. Since this configuration may not be ideal for other applications that may be put within our environment, we our entertaining the idea of isolating SharePoint into its own instance, probably on the same box.

My manager wants me to come up with performance trace data to better prove that we need to go this route since we apparently have had issues in the past by blindly following Microsoft's best practices.

1.MAXDOP configuration - I understand this may be a 2 pronged approach that would require looking at various execution plans and CPU related counters in Perfmon. SharePoint likely requires a maxdop of 1 due to the nature of the application (lots of concurrent processes). What is the best way to show this need graphically?

2. Memory configuration for multiple instances - Does the Total Server Memory reveal all the memory that a given SQL instance is utilizing? Should I use this counter to identify appropriate min/max memory configurations for multiple instances on a single cluster?

The problem with the perfmon approach is that it's scope is limited to just the server. Since our SharePoint environment is currently being shared with other applications, I understand that I may have to utilize DMV statistics to narrow down my analysis.