Applying SS to Requirements Mgmt (CMMI)

I have read lots of information regarding the complementary nature of Six Sigma and CMMI and agree that they can be used together. 

We are currently implementing CMMI KPAs and would also like to institute Six Sigma Process Management systems for each KPA.  I'm trying to figure out the best approach in keeping measures of our Requirements Development and Management processes.  I have brainstormed the following

• # of changes to requirements
• # of requirements not successfully implemented
• Time spent to develop requirements
• Some measure of quality of requirements?

Any thoughts would be helpful. Thanks