Shadow Blades is still best used during Adrenaline Rush, because it buffs your auto attack damage while also giving you additional combo points for more eviscerates. It's just a bad set-bonus for Combat.
Faster playstyle - more spamming; for assassination it's a sensible thing, for combat it could lead to energy capping due to the use of AR + SB over an increase of haste. Not completely sure if it's a risk for sub too, but it doesn't seem the case.
Dps increase? Check. Not a good mechanic anyway - it's a replica of the TotT set bonus, just with a longer CD and a bigger effect.