I am thinking about going back and re-beating Deus Ex: Human Revolution - Director's Cut for an achievement run, including The Foxiest of Hounds and Pacifist achievement. I am wondering if playing pure stealth and never getting detected would be any harder on the hardest difficulty compared to the easiest difficulty, so I could also get the achievement Legend. I imagine bosses would be harder, but would the normal part of the game be any harder?
It makes the game harder because you'll be paying attention all the time to everything so that you won't be forced to start again a specific section. You'll save often.
The Pacifist run will make the game about 20% harder, because you'll be forced to use the stun gun that has a limited range and the tranquillizer gun that requires a bit of planning comparing it to the sniper.
Also, saving Malik will be harder (no killing of any soldiers), the zombie-people situation at the end, the hostage in the first part of the game etc.
You'll stock on non lethal weapons and focus on punchy-sleepy takedowns.
The Deus Ex Machina difficulty level won't change much considering the above statements. Enemies will kill you faster because of their increased damage, but if you're going for the The Foxiest of Hounds achievement you won't really care that much, you're sneaking and won't be in a situation for the enemies to fire at you after being detected (you'll quick load anyways when that happens).
The boss fights are the only thing to consider, but you can definitely kill them easy if you're stocking in stun gun ammo, have a fully upgraded lethal weapon and/or tyhoon ammo (2-3 blasts per enemy and they're gone). In the Director's Cut version, boss fights were re-thinked and as far as I know, you can now use Hacking to turn turrets/robots on them.
In conclusion, this could be the best to play the game. You'll be fully aware and the game will be moderately challenging.
Follow this as a guide to check if you won't miss any Smooth Operator bonuses: http://deusex.wikia.com/wiki/Foxiest_of_the_Hounds