What is the real effect on the bottom line?

Six Sigma Quality is often touted on the basis of its significant bottom line impact. Some claim more than $1M per year per Black Belt in typical cost savings. For example, according to one Motorola Six Sigma Presentation, in 1996 they achieved 5.6s performance (up from 4.2s in 1986), $16B in cumulative manufacturing cost savings and a reduction in Cost of Poor Quality from 15% in 1986 to a little over 5% of sales in 1996. I'm not sure where that number comes from nor where the billions of dollars in resulting claimed savings went, but I'd really like to see an independent audit so that I could understand the basic assumptions used.
I would hope that the calculated savings net out the component of traditional cost reduction, as captured, for example, by the historical cost experience curve, so that the resulting number is truly reflective of the incremental savings that are directly assignable to the 6s initiatives. It is always very tempting to attribute all benefits to the current program, regardless of their true origins.