Going from some info I have picked up along the way and some reading of
"Engine Management: Advanced Tuning by: Greg Banish" I would say the AFR you are looking for is around 17.64:1 (lambda 1.2) for close to the same emissions as running at 14.7:1.
For cars that have a issues when going to/from lean burn aim for 15.43:1 (lambda 1.05) according to above book. I have not finished looking over the book but just to let you know there is not much of anything on lean burn in it.