Here's a post I wrote on another forum. I think it applies to your car as well.
How lean you can go depends on how much airflow (grams per rev) is going through the motor. At light load you can go very lean without fear. As airflow goes up the A/F ratio should gradually get richer until around 5 in/hg vacuum where it should be around stoich.
From my experience the limit to how lean you can go will be misifires. If the motor misfires, it's too lean. Bigger plug gaps helps to reduce lean misfires, but you'll have to balance that against WOT misfires if the gap is too big. I've been able to run slightly leaner than 18:1 at very light throttle.
I also found that my motor doesn't have a smooth idle and it stalls easily if the A/F ratio goes leaner than around 16.0:1. So I've set my rolling idle A/F ratio at 16.0, then it goes to 18.0 just off idle, and gradually richens.
A side effect of running leaner than stoich is that the mixture burns slower, so knock is less likely, but the engine needs more timing to keep power levels from dropping, so I've also added one or two degrees of timing in the same parts of the map that are very lean. It's working very well!
There's some good info in this link.
http://www.max-boost.co.uk/max-boost...ion_deeper.htm Scroll down to the bottom, there's a little chart that shows mixture burn speed at different A/F ratios. It'll give you an idea of timing changes needed for leaner A/F ratios. The whole site has plenty of good reading.
HTH
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