People were hitting 425W using a modified BIOS with a reference 980 Ti.
 
Increasing the power limit alone can't damage the GPU.  If you add voltage on top of that, then you can decrease its lifespan.  But power limit alone will not affect the life of the card in any significant way.  How high the power limit can go is limited by the BIOS and shunt resistors, so if NVIDIA didn't think it was safe to increase the power limit then they wouldn't allow it.

hmmm I see some conflicting statements. Quad5Ny said it is fine to turning voltage all the way up, ArmeniusLOD said otherwise. 
 
@ArmeniusLOD: regarding Nvidia "allowing" it, I would assume that anything we do with OC tools is kind of a gray area between allowed and not-allowed. If it was safe, I suppose the increased power limit would be the default, instead of requiring the use of tools that go beyond the Nvidia control panel. This is exactly what keeps me wondering why it is not the default.

It's not the default because NVIDIA tests their products within certain tolerances and that is what you're guaranteed out of the box.  What they're doing is trying to get as close to 100% usability of the chips that are being produced.  A good proportion of the chips they make are probably stable at 120% of the default power target, but the amount that are not can result in returns and damage to the confidence consumers have in their products.  It can often take only a handful of loud voices to bring a company down.  At the same time, a large amount of people will probably decide not to purchase a video card with no overclocking ability at all.  They also have to keep TDP within the specifications of the cooler they are using.  If they increased the power limit by 20% by default, then that is an extra 50W of heat that needs to be dealt with to stay within tolerances.
 
This is just my armchair analysis.  I'm sure there are plenty of complexities that go into bringing a product to market that not everyone can fully understand from a consumer perspective, especially a complex piece of computer hardware like a video card.

sniperganso
hmmm I see some conflicting statements. Quad5Ny said it is fine to turning voltage all the way up, ArmeniusLOD said otherwise. 
 
@ArmeniusLOD: regarding Nvidia "allowing" it, I would assume that anything we do with OC tools is kind of a gray area between allowed and not-allowed. If it was safe, I suppose the increased power limit would be the default, instead of requiring the use of tools that go beyond the Nvidia control panel. This is exactly what keeps me wondering why it is not the default.

In some respects it's a marketing gimmick. You're right they could just have the default set at the max. But being able to 'boost' the GPU is a selling point.  Gives the user something to play with, along with the clock sliders.  Too answer your question, anything you do software wise with tools like Afterburner or Precision have no affect on your warranty with EVGA because you're still running within the limits set by the stock BIOS. For Pascal, temps are probably the most important factor in both longevity and performance. The lower your temps, the higher and more stable your boost clocks.