Dunno about the supposed ott´ness of DW Marines. To me they don´t seem so.
A marine is in every way above and beyond DH acolytes. Just like an officio assassinorum assassin (like a Callidus or Eversor assassin for instance) is above and beyond a DH acolyte assassin.
I also see no issue with the wargear (other than internal balance issues with DW wargear maybe, like standart bolter vs storm bolter, etc.). Yes an astartes bolter packs more punch than one designed for regular humans (and the DH stats are craptastic, even when compared to a las gun, I admit), so what? The thing is larger probably for reasons other than fitting proportions (as in not looking strangely undersized).
Imo DH characters don´t have to and should not match with space marines (not one on one anyways) but that´s just me. Yes there are Inquisitors (not acolytes) who even wear terminator armour. Those armours are millenia old relics, everyone a unique masterpiece, which probably can´t be recreated and only a handful of them exist in the whole Imperium. Those are similar to astartes terminator armour in terms of portection, etc. But unless you´re one of the most powerful inquisitors in the Imperium you won´t ever see one of these.
Regarding the differences between regular marines, death watch space marines and grey knights. I think things get a bit blown out of proportion here.
Death Watch space marines are "just" veteran space marines. Take the first company (or equivalent formation in non codex chapters) of any chapter and you´re good to go. Death Watch marines in terms of power are veteran marines, period.
Then you got the Grey Knights, again they are not much more powerful than regular marines. They are psykers, but the regular grey knight is not on the level of a Librarian, so no "OMFG imba kickass warrior-wizard-paragon of awesome" if you get the drift. Most are merely able to perform psyker abilities as a squad (libbies do that on their own).
They do have better equipment. Where the regular marine has a bolter and a combat knife, the regular grey knight has a wrist mounted storm bolter and a nemesis force weapon. Their armour is of higher quality too. No better protection vs normal attacks but more extras and something vs psker attacks as far as I recall. And last but not least their training standarts are higher.
Basically a grey knight vs a normal marine is like a special forces soldier vs a battle hardened regular trooper.
And lastly lets not forget that unlike the TT, in an RPG we are able to portray individuals. So in the lore, there are awesome warriors who are just regular "rank and file" by rank. The sergeant is not autmatically the best fighter, and a DW, or GK marine is not automatically > than an a regular one. On average, yes, but the differences are much smaller than some people make em to be. Most of it is "know how" and experience. GK´s know how to deal with deamons and how to resist their influence better than anyone else. Same with DW vs aliens (especially against those which are not as well known to the imperium as, l et´s say the orks are).