Unless the "C" model you were looking at had an O-320 engine in it, there shouldn't be so much performance difference between it and a "D" that you'd notice.
The book numbers reflect mainly the increase in gross weight from 1500lbs to 1600lbs. There were other differences due to the rear-window drag and a prop change, but loaded to the same weight, both planes should perform just as anemically at 7,000'.
