As you say, it does depend on how much the subject is moving, the OIS takes care of camera movement very well.
For gentle wind movement, 1/125 seems OK most times, and a short burst at 1/250 as you mention will usually get a good shot. (one shot will hopefully be at the end of a sway, when there is very little movement)
Haven't tried the 100-400 much since the days have got darker, the 100-300 is much nicer to use
If using JPEGs try the settings I posted the other day, and you will probably find you can use a higher ISO too.