The descent stage already weighs almost twice that of the aeroshell so it shouldn't really affect the TWR or fuel needed.
What is the point of having different stages for different parts in the mission? Wouldn't it be easier to add retrorockets to the aeroshell and cut the parachute? The descent stage already weighs almost twice that of the aeroshell so it shouldn't really affect the TWR or fuel needed. Furthermore, this has already been used three times (albeit without cutting the parachute) on the two MERs, Spirit and Opportunity, and also Pathfinder, so the technology is mostly there.The benifits could also include more space in the vehicle and/or more fuel for landing.
I just don't see why they can't integrate the skycrane into the aeroshell. Fewer points of failure and theoretically more simple. Just like the MERs but probably needing to separate the parachute just prior to powered descent, just like they would have done anyway.