Once Hypervoice (175BP after ability boost) is available, I think Sylveon could become one of the stronger special attackers in Gen VI, somewhat comparable in speed, and more specially bulky than Magnezone of Gen V, with Sylveon dealing more damage despite sporting 20 less SpA as long as it's using Pixelated Hyper Voice. Unlike M-Gardevoir, Sylveon can hold an item while having access to Pixelate, and is slightly bulkier (but obviously slower). Theoretically, it could boost its damage further with Choice Specs, and with SR/Spike support, there are few things that can avoid a OHKO from Hyper Voice short of special walls and those that resist Fairy. Alternatively, Choice Scarf makes it a fairly bulky Special Sweeper and makes up for its lack of speed.
move 1: Hyper Voice
move 2: Psyshock
move 3: Hidden Power Ground/Hidden Power Fire
move 4: Shadow Ball
ability: Pixelate
item: Choice Scarf
nature: Timid
evs: 4HP / 252 SpA / 252 Spe (same speed as Scarf Magenzone)
Alternatively, the slower, bulkier, specs user:
move 1: Hyper Voice
move 2: Psyshock
move 3: Hidden Power Ground/Hidden Power Fire
move 4: Shadow Ball
ability: Pixelate
item: Choice Specs
nature: Modest
evs: 252 HP / 252 SpA / 4 Spe (again, some EVs should be taken from HP to Speed to depending on comparable speed tiers)
Hidden Power Fire would only be for prediction against a switch-in Scizor as there's no way you can stay in if it comes out, or ground for other Steel/Poison type threats, but for the most part, this set would be sticking to Hyper Voice spam, as it's the only move that gets boosted by Sylveon's ability (even resisted, it's stronger than your other options if they're only neutral).