In my experience powders burn cleaner up near the top end loadings.
I have never shot 11FS but shooting Unique, 231, Blue Dot, Red Dot, and a host of others at lower charge weights results in massively dirty cases and guns. As you bump the charge weight up, the powder gets cleaner and cleaner until you just have some light gray dusting at the muzzle and around the cylinder gap.

In the end I have found that it is better to pick a powder that is providing the velocity you want up near 80-100% of the max charge weight. If you need to download then pick a faster powder.