First, it's usually best to max out 2 stats instead of spreading EVs around. Second, focusing on one attack stat has definite advantages. Using only one attack, you can max out that attack stat and another stat (such as HP or speed) and get a higher chance to take a hit or outspeed the foe. Another advantage is explained best by this amazing database:
>In most cases it is preferable to have a nature that decreases either Attack or Special Attack for Pokémon whose strengths are the opposite type of attack. Espeon, for example, favours Special moves, so it's best to use a nature that decreases its Attack since it won't be used.
I'm not an expert at this, but for fast sweepers, (I think) you should have a nature that boosts speed rather than either attack because (I think) increasing the chance of outspeeding a foe is more important than increasing the chance of OHKOing it.
For Gardevoir, you'd want 4 Def, 252 SpA, and 252 Spe. You should have trace so you can see what the opponent has before mega evolving. You should also replace modest with timid and replace thunderbolt with psychic since it doesn't cover any weaknesses.
For Arcanine, you'd want 252 Atk, 4 Def, and 252 Spe. Replace brave with jolly, and heat wave with flare blitz.
For Goodra, you'd want 252 HP, 4 Def, and 252 SpA. Replace quiet with modest, and replace earthquake with dragon pulse.
For Zoroark, you'd want 4 HP, 252 SpA, and 252 SpD. Replace modest with timid.
For Vaporeon, you'd want 252 HP, 252 Def, and 4 SpD. It isn't really going offensive, so replace modest with bold for more bulk, muddy water with surf for better accuracy, ice beam with wish for healing, and signal beam with heal bell for toxic prevention.
For Ferrothorn, you'd want 252 HP, 4 Atk, and 252 SpD. Replace iron defense with curse because it raises physical attack and defense while lowering speed, increasing its ability to take most contact moves and further increasing damage from gyro ball.
