Scarab swarms are an interesting topic. They have a few special rules that make cover saves complicated. There is a Scarab swarm special rule called small target, this rule states that a scarab swarms cover save is given a +1 modifier because they are hard to hit. The 5th rulebook has special rules for swarms, and this states that swarms have the stealth rule. Stealth also grants them a +1 modifier on cover saves.
Now the question is do scarab swarms get both those +1 modifiers or not? The Necron faq (around the bottom of the second page) clearly state that scarab swarms are to be considered swarms. Should they get both +1 modifiers it would only affect a few select situations. Because you can't have a 1+ cover save (meaning you would always save) when you turbo boost the save is still a 2+ cover save.
What would be affected is when the unit of scarabs and any independent character attached to that unit are in cover. If the terrain in question grants a 4+ cover save it would be counted as a 2+ if both rules were used. I wont go into detail about the 5+ or 6+ cover saves because I'm sure you can all determine what they would be.
I don't know how to take this exactly because there is some overlap with other rules for the scarab swarms also, but they do exactly the same thing. The rule vulnerable to blasts is given in both the necron codex and the 5th edition rulebook though this is the exact same rule, granted the stealth rule and small target do the same thing but at least have different names.
Personally I will be talking with whoever I play against before hand to determine how this rule will be played during the game. I suspect that if and when the Necrons get a new codex that this will be taken care of but until then it's basically between the players how the rules should be interpreted.