Even then, Echoed Voice just isn't worth. You basically HAVE to use the move next turn to take advantage of the now "Hyper Voice" move. Steel has remained popular since the GSC days and with the introduction of some viable Fairy-types (Azumarill, Togekiss, etc), Steel-types like Scizor are going to stick around. Meaning they will switch in after you take out that Dragon.
Gardevoir wouldn't be doing anything to Steel-types to begin with, Echoed Voice or anything else. At most she can take off a chunk of HP with Shadow Ball (or try to KO with Focus Blast if you like running that), but Bullet Punch Technician Scizor will ALWAYS OHKO Gardevoir. Keeping her in against Scizor(or most steel-types in general) is suicide no matter what set you run. Better switch out to a Fire-type or a physical wall.
And even if you don't, the other opponent will be at an advantage whether they decide to sack their Dragon or not.
I'm not going to put down calculations, as I'd likely mess them up regardless. Multiscale Dragonite aside, Echoed Voice at a mere 40BP can OHKO a Dragon-Type pokemon due to stab, pixilate's 30% boost and her 165 base SpA. Garchomp has a tiny chance of surviving, but that's it (And Goodra, being a special tank, takes it with ease). And this is not counting prior SR damage or any Calm Minds. It is more likely for you to beat the opponents dragon than it is for them to survive.
Don't get me wrong. The moment I can put Hyper Voice on my Gardevoir (which for me will be until a Tutor in XY's sequel comes around, as I'm not retraining a Shiny 5 max IV Gardevoir on an older generation), I assure you, I will. It's much safer. That doesn't mean Echoed Voice isn't worth it for now, however.
(All of the above is based on in-game battles, although my "these-may-be-wrong"-calculations
generally match up.)
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Gardevoir@Gardevoirite
Modest
4HP | 252SpA | 252Spe
-Psychic / Psyshock
-Will-O-Wisp
-Calm Mind
-Moonblast/Hyper Voice/Echoed Voice
That's my set, and it's been working out so far. It lacks coverage, sure, I am aware of that, but it can take even physical attackers (bar Bullet Punch Scizor) with ease thanks to Will-O-Wisp. Psychic is generally preferred as it deals more damage, although Psyshock could help take down things that are more specially defensive. Moonblast is the safe option for Fairy STAB, until Hyper Voice is available, after which Hyper Voice is generally the best option. Echoed Voice can still hurt and KO most things weak to fairy, despite only 40 Base Power, thanks to Pixilate, and reaches Hyper Voice's base power after a single use. It can be tricky to set up, should the opponent be running anything with priority, but due to capping at a whopping 200 base power before Pixilate, may potentially sweep far better than Hyper Voice.
Main checks and counters are of course, Steel Types, as they resist anything this set can throw at them. Sticky Web support is almost mandatory despite having a very respectable 100 Base Speed, to ensure Gardevoir always goes first (not counting priority). Stealth Rock is nice to have on the field, but not a necessity.