ok here is the study. it was done with extremely low birth weight infants. the study found that bottle fed infants gained faster, but breastfed babies exhibited less signs of stress during feeding, as measured by things like oxygen saturation, heart rate etc.
the conclusion was that even extremely low weight infants should be breastfed but with breastfeeding support for the mom and possibly supplements as needed to optimize weight gain. http://pediatrics.aappublications.or.../100/6/e3.full