Here is an answer that I posted in another thread about SLI temperatures but it still applies in this case.
I can assume that a more stressful environment would lead to shorter life expectancy of any given component.
The "ideal" or safe temp for a GPU would be anywhere under 80c, I would lean more to the 75c average. When cards get hotter than that they can suffer damage to the PCB and warp over time due to the severe temperature swings. If you approach the 90-100c mark the card can "burn out" this can be anything from melted and warped parts or if you are extremely unlucky a fire. The odds of getting an actual flame are very very low but it has happened.