Some really well made interesting points in this thread from different points of view
I agree that DH has been massively influenced by Call of Cthulhu and the focus is on investigation.
Not sure that I would necessarily call it film noir as such as I think the characters are more intense and driven rather than the ambivalence that characterises many film noir characters. There is also far less chance of romance or simply erotisim being present in a typical DH session than is a key feature of film noir. Though maybe I am playing with wrong group lol (I have images now of everyone else playing in groups which have at least one player who is a modern incarnation of Rita Hayworth or Marlene Dietrich ). That said I do know what your getting at if the film noir reference. DH is certainly allows the PCs to play anti heroes and even villians (albeit ones facing worse threats).
Where I would disagree is that I think DH also allows for a genre that is outright action. Sure things like melta guns and bolt guns should be out of reach for players most of the time but just look at die hard or Lethal Weapon. Those franchises had weapons that barely went beyond autopistols and replace Street hoods and European terrorists with Hive gangers and chaos cultists and you have some pretty interesting DH sessions right there.
With regard to the OP given that I am the kind of GM who is happy to throw a Eversor Assassin at rank 2 or 3 players then I thin it only fair that NPCs don't scale unless there is a specific reason for them to. What you can potentially change is their weapon loadout which could change in response to the PCs.
You can also use a system of scaling which isn't too unfair and represents ability
so a 'experienced' bad guy might have +5 Bs OR Ws and +3 wounds
'Veteran sgt level badguy +10 Ws OR Bs and +5 Ws or Bs +3 wounds +5 Ag
Lt or 'champion' level bad guy +10 Ws and Bs +5 wounds and +10 ag. Possibly one or two extra talents
I would give the PCs fair warning though, i.e maybe wearing different uniforms or obviously giving orders. I wouldn't simply have all the hive gangers on Volg suddenly gain a level in badass.