I usually see light-heavies win the overall at NQ shows. However the heavyweight won at the last NQ show I did this spring, and then went on to win junior nationals.
I agree that the superheavyweights are often not in as good shape as the lighter weight classes, but not necessarily because they insist on being superheavyweights. Some probably just carry too much offseason BF to get ripped in time.
Many SHW also have the disadvantage of being rather tall, which makes it more difficult to present a thick, balanced package. The SHW I've been up against have had both problems. One was so out of shape he knew he was screwed and went out drinking margaritas in between prejudging and finals.