I ran into this same problem again today.
The problem with the progress bar is that the sliced sprite's minimum width keeps it from smoothly , since it jumps from 0 to the slicedsprites minimum width. Filled sprite can't be sliced or tiled, which limits the sort of assets you can use for this. The only way this works is to hardcode a sprite to the exact size you want, or use a slicedsprite that doesn't have any sliced sides in the direction you want the progress bar to flow.
I for one would love to see fillsprite become something other than a sprite type, to be applied to any sprite.