Ubuntu 16.04 Blender 2.78
I'm attaching a blend file.
Using the Soften brush on high color depth images totally ignores the strength and curve settings of the brush. No matter how you set it up, it blurs the image in full strength and the boundaries of the stroke don't get smoothed out.
In the attached file you have a cube, the brush is set to 0.1 of strength and the curve to smooth. In the image slots of Texture Paint mode you have two images, 32bit and 8bit. Just draw a couple of strokes on each one and you will notice the difference.