I had the same issue with my H70, there is a curve to the surface. This cause my TIM to pocket in the middle. I noticed it right away by monitoring my temps. My average temp was way up and simple things like opening a web page would spike the temps by 10c or more, took longer than normal to cool down also. When i removed the cooler from the chip (Nvidia card so i had previously removed the IHS) there was not a spot of TIM in the middle of the chip, all had flowed to the edges. Ive ran the mod for close to a year and ive used the same cooler on a 560ti previous to my 580. Ive not had that happen befor or after. **** happens, what can you say..right. Redo the card, if the pump slides even a little bit during the install you will have to redo the TIM again. Ive done the mod a dozen times over the last year and never once has the temps not been affected if the Tim gets smuged. The point where you connect the card to the pump is the most important step and great care must be taken during this step to ensure a proper contact. Smuging the TIM is worse than having the zipstraps too lose.
I will use plyers to hold the extra zipstrap end that has been fed through the pcb holes NOT the knuckle ends used to "lock" with. Ive been afraid that if i slip ill scratch/chip the pcb. By holding the extra zippys coming through the pcb you can actually stretch the zipstrap a little bit more allowing for a few extra clicks on the locker piece. This all so adds to increased pressure because when you let off on the plyers the zipstrap will squeeze back to its original lengh pulling the pump to the chip that little bit extra.
I also only put TIM on the chip, not the cooler. There is less guessing when trying to line them up. Helps too if you have a extra set of hands to hold the card and pump while the the zipstraps are being tightened. Im due for another redo soon, youll know when your temps climb that little bit extra over what they normally do. Im now creeping upwards of 55c when my normal is 48c. I also average about 18-19c on startup and idle at 21-22c now im 22c for startup and idle at 27-32c. Great temps for sure but this is the point where you know its time to redo your TIM.