What are the benefits pursued with the revision of control plans when our process shows high values of cpk (>=3), as is suggested in point 4.9.2 Maintaining Process Control ?

What is your Ppk?
With the Cpk at 3.0 it shows the process to be stable & centered. Your charts probably shows the contol limits spread are probably around 20% of the Tolerance.
Benefits for changes. Decrease inspection frequency for labor costs. Just make sure that the output that has Cpk >3.0 runs at the 20% level of Tol.

For a good thread on Cp, Cpk, Ppk, etc., see:
www.16949.com/ubb/Forum10/HTML/000001.html (http://www.16949.com/ubb/Forum10/HTML/000001.html)