Greninja already outspeeds the majority of the OU tier, and there are better Scarfers out there.
Scarf Greninja outspeeds all common Scarfers, and unless the Scarfer in question is Rotom-W chances are it is going down pretty hard.
Choice Scarfs are becoming increasingly rare, as is boosting sweepers. A Scarf does not predicate a revenge killer, don't use your connotation for the definition. If Greninja can switch in on and then KO a Pokemon threatening to sweep your team then it is a revenge killer. Try and run Dragon Dance Dragonite and expect to get through Talonflame, then tell me whether or not a Scarf makes a revenge killer.
Greninja has awesome coverage, unlike Talonflame, allowing it to revenge kill many different Pokemon. It also doesn't lose a third of its HP every time it deals damage. Also to be fair:
252+ Atk Choice Band Talonflame Brave Bird vs. 4 HP / 0 Def Dragonite: 246-289 (75.9 - 89.1%) -- guaranteed 2HKO
Scarf Greninja can revenge kill almost any setup sweeper unless they get to +2 Speed, it can revenge kill almost every Scarfer there is, and it can outspeed +2 Speed Cloyster.