This is a tough question that Billy has posted as Maury River MS prepares to move into homogeneous grouping of students, so I am going to put this out for all of you to think about. We still have the common goal of deeper understanding for any student, no matter what the ability level of the student. The amount of scaffolding needed to get the student to that "deep end of the pool."
Billy correctly identifies that somewhere in the UbD framework there needs to be an assessment as to where the student currently resides academically so a starting place can be defined, because even within those "homogeneous" groups you are going to have learners with different holes in conceptual knowledge and/or thinking and processing skills.
So I think as you are assessing "what students need to be able to do and understand" you have to look, not only at the ending outcomes of the unit of study, but look at how to do a pre-assessment activity that can assess whether or not they have the prerequisite skills they need to do the first couple of activities in the unit.