For Lights? Sigma High of 6 is kind of high. What was Sigma Low? The default values are above.
If they are too high or low you will not eliminate noise or eliminate valuable data.
That's not particularly high for Winsorized sigma clipping. I typically start at 7. WSC works much differently than regular sigma clipping. In regular sigma clipping, the outliers are included in the calculation of the standard deviation (sigma). If there are bright pixels in the pixel stack, the resulting sigma will be much larger than that of the underlying no-outlier distribution. As a consequence, it takes a small sigma multiplier to reject the outliers but not the underlying distribution. With Winsorized sigma clipping, the data set is Winsorized prior to calculating sigma. Basically what Winsorization does is replace the outliers with the closest inlier. The resulting sigma value is much, much smaller. As a result, it takes a significantly greater sigma multiplier to reject outliers yet still retain the underlying distribution.
The result of choosing a sigma multiplier with WSC that is too small is that too much of the underlying distribution gets rejected. This is particularly noticeable around bright stars. It is the result of star sizes in the subs that vary in size due to focus, seeing, guiding, etc. In that case, the regions surrounding the star core will have pixel stacks that are bimodal - some are dark background, and others are much brighter, with star stuff in them. When you apply a too-small sigma multiplier to a bimodal distribution like that, a lot of the bright pixels will be rejected, making the pixel much darker than a plain mean of the pixel stack would produce. The result is dark splotches around the cores of stars. Those are the results that are obvious. The less obvious result of a too-small sigma multiplier is increased noise in the darker areas as too much of the true underlying distribution in the pixel stacks gets rejected.