No.
If both Pokémon had 252 EV's in Speed, 31 IV's in Speed but only Gengar had a Speed Increasing Nature, Gengar would out speed Greninja unless Greninja had a Choice Scarf or received a Speed Boost of some kind.
Greninja with 252 EV's in Speed, 31 IV's in Speed, and a Modest Nature = 343
Gengar with 252 EV's in Speed, 31 IV's in Speed, and a Timid Nature = 350
Source: Showdown
