I’m starting with BS too because of the possibility of correcting EPI distortions with T1-weighted images.
I used both the –dwi-mask and –t1-mask in order to improve the correction following your advice, but the outcome was pretty the same. In fact, the outcome was even worse when I used one (–dwi-mask) or both masks (dwi+t1) than when I did not use any of them.
Maybe without a fieldmap and only a T1 little correction could be made? Don’t get me wrong, I’m quite glad with correcting at least a little of such distortions than leave my DWI volumes with such disturbances.