Seems to me that you have described a traditional quality improvement project.
Based on the elements of your question, it is my understanding that:
1) Each “drawing feature” constitutes a “defect opportunity.”
2) The customer’s minimum expectation was a quality level of .99, per feature.
3) The pre-project quality was found to .995, per feature (opportunity).
4) The post-project quality level was found to be .998, per feature (opportunity).
5) Your short-term benchmark was determined to be Z.bench = 4.9.
6) You wonder if it is possible for “Six Sigma” to assume a value other than 6.0s.
Given these facts, I have several comments:
1) A quality level of .990 per feature translates to 3.83 sigma.
2) A quality level of .995 per feature translates to 4.08 sigma.
3) A quality level of .998 per feature translates to 4.38 sigma.
4) You have assumed that the defect categories are independent.
5) The “benchmark sigma” of Z.bench = 4.9 does not equate to 4.08.
6) Six Sigma means Six Sigma, not Five Sigma or Four Sigma, but Six Sigma.
In summary, I would say:
1) The accounting of “opportunities” is likely to be inconsistent with your definition.
2) The assumption of independence is crucial to your accounting and computations.
3) Six Sigma is not a “rubber ruler.” One should not stretch the numbers.
4) Your efforts are consistent with that of a “qualilty project” (not a Six Sigma project).
5) You made a 2.5X change in the quality base (given the stated dpo levels).
6) Do you really understand what “short-term sigma” means?