First of all I would like to convey my heartfelt thanks to all of you and this AnandTech community.
Actually I sometimes believe that as @Seba had suggested that it could be due to badly applied TIM or some other hardware issue as the card which I got was manufactured in November 2014 and was lying unused in the warehouse of the distributor for around year. That's also the reason i got it for cheap, around 130$ less than market price. That period might have had some effect on card or TIM, I don't know.
Also I had not made and changes in clock speeds and anything, I was running the card on factory settings. Its factory oc'd t0 1070 MHz.
So Followed the advices of all of you and ran the tests once again and the method I followed is as below:
First of all I changed the power plan to high performance.
Then went to BIOS settings and changes minimum cpu fan speed 100% and disabled C, C1E, C3 and C6:
Then increased the power limit in CCC by 20%.
Changed the GPU fan speed to 100% in Afterburner.
And finally ran the Heaven Benchmark.
CPU Details while benchmarking:
GPU Details while benchmarking: