Using Central Management Server to provide convincing performance statistics
How can I, only using Central Management Server, provide convincing
  performance related information for the individual servers managed
  with it?


Unless I've missed a key feature somewhere along the line, you can't. What does it give you?


Logical organisation of your servers and instances.
Policy Based Management can be applied to server groups.
Multi-server queries.


For performance data you'll want to look at installing the Management Data Warehouse alongside your Central Management Server. It isn't as useful as the various 3rd party tools for collecting SQL performance data but it's an improvement on roll-your-own Perfmon/SSRS/Excel solutions.


  I would like to use Central Management Server to go one step further
  by going to our customers and saying, if you did "this" then your
  website or application would be faster times X.


If only it were that simple. You're going to be on shaky ground if you offer this sort of advice to customers, especially if you're sending them a bill for the proposed upgrades. You can certainly offer some insight but not guarantees:


ServerX has a low page life expectancy, it would benefit from additional RAM.
ServerY has increased from an average 50% to 70% CPU in the past month. Has anything changed or have you seen an increase in visitors/customers? Perhaps you should plan for an upgrade.
ServerZ used an average 500IOPS at the start of the year, it's now reached 750IOPS.