Guys we need to remember that BWL and particularly MC were raids specifically designed for people who had never raided before.... As previously stated they are now raids private server players have cleared on multiple differing servers multiple differing times. 
  
AQ40 saw the introduction of fights that incorporated more than one boss to give raiders experience in raid cohesion and add a new element to raid dynamics... 
  
Naxx then took the difficulty to a whole new level, for the time raid mechanics but another factor was the unforgiving nature of those mechanics.... for example in the early wings of Naxx take Maxxenas webbing (if people were not 80% health + when webbed = dead) and Heigans eruption (= dead if touched outside of MT). this unforgiving nature had never been experienced before... 
  
Jes i still remember my retail guild wiping on the first trash mobs in Naxx omw to Inst Raz (the gargoyles) , a guild that had been clearing BWL and clearing to Cthun for months and we still wiped on the first trash mobs... Imagine how that felt 
  
The jump in difficulty between BWL and AQ up too C'Thun and Naxxramas cannot be understated and hopefully its replicated here because it has not been replicated on any other private server 
  
EDIT: So in short if Naxx is scripted correctly there is no need to add additional mechanics / HP to increase its difficulty