I always use the Pentax IBIS for stabilization. I have played with the Sigma OS but have been using in body so long I am accustomed to its quircks.
The various settings for the Sigma OS are off, normal stabilization, and stabilization for panning (eliminating one axis of stabilization).
I have struggled with sharpness, and really put it through its paces shooting sports and birds. I would offer that wide open the 120-400 is just plain soft. Further to that accurate focus can be a real trick. I have not posted much in this thread, but i'll add a few pics for you.
For example - hand held - f11
Heavy crop out of the centre handheld @ f8
handheld @ f8 with some OK light
Use Pentax TAV mode and don't be scared of higher ISO's - the faster shutter, and stopped down aperture will yield better results than shooting slow.
@ 5.6 but 1/800 shutter speed.