Nighthawk X6S

Can you install it that way? Sure

Will it potentially reduce the lifespan of it? Probably. Its designed with a certain orientation to passively cool it. If you're laying it down/putting it on its side, its potentially impacting how it cools. heat is the death of electronics. 

But it might never impact it or cause an issue. 

For example, I have a RAX200 thats not designed to be wall mounted. But it is. And I've had it over a year with it running normally and the fan never kicking on. 

So its kind of up to you if its worth the risk of a shorter lifespan or not. 

Or if you've got a fan/cooler to direct at it, it might not change it at all.