I travelled from Lahore to Sukkur in less than 6 hours (from Lahore airport to Sukkur toll plaza) with an average speed of 135kmph. I wonder if you are telling the truth about 180kmph average speed and still reach Sukkur in 8 hours.
To @sebali, the original poster: It is easily doable in 13-14 hours with a couple of small stopovers and not stretching the speed limit. You shouldn't and practically couldn't go beyond 120kmph (average) from Hyderabad to Sukkur as it would be too risky given the road condition, traffic (tractors, trolleys, Ching Chee, motor cycles, rickshaw and what not) and also towns every now and then. However, you can easily maintain an average of 130kmph on M-5 (Sukkur-Multan) and then M-4 (Multan-Abdul Hakeem) and M-3 (Abdul Hakeem-Lahore).
I am not in favor of over speeding to put one self and others' lives in danger. What would reaching an hour or 2 early do to your life? comparing what it can do if God forbid anything bad happens?
Broadly speaking:
From Karachi toll plaza (M-9) to Rohri (M-5): 6:15 min (with one stop of 15 minutes)
Sukkur-Multan (M-5): 3.45 min (one stop of 15-min)
Multan-Lahore (Ravi interchange): 3:30 min (non-stop)