My 2 cents. Yes, I consider it a form of "cheating". Why? It's not an in-game option, so it was not intended to be altered by the end user. No matter how easy it is to do.
In a FPS game, one of the biggest advantages anyone has is "battlefield awareness", and FOV is a huge part of that. If I edit my game to set my FOV at 90+ and most other players are at default, I have a distinct advantage over them. Pure and simple. The only way to compensate for it would be if everyone had it set to the same value.
You can't compare it something like Nanovision since that's a game feature and everyone has it.
These types of edits are far to common in online gaming. I know many people to edit their games in order to increase visibility to give them an advantage over other players. From increasing FOV like whats being discussed here, texture modding, advanced engine settings like you can do with Unreal Engine powered games. In most all cases the effect is the same, you can see more of the battlefield than other players, and in most all cases it's considered cheating by the game developer/publisher or server operators.
The most I will do to gain a competitive advantage in an online video game is adjust my graphic settings to remove as much clutter and distraction from the environment as possible. Many competitive gamers do this, but they do it within the games available settings, they do not edit the game files in any way. Where as many people always try and play the game at it's maximum graphic settings. The guy running at lower settings but high resolutions often has a tactical advantage since he has a much better chance of seeing you before you see him.
Well said and I agree.