You should definitely be running a Timid nature with this guy. 405 Speed is such a good number with all these new threats running around, such as Mega Gengar and Talonflame. Using a Modest nature sorta takes away the whole point of using him--outspeeding basically everything that doesn't have a Scarf. Anyway, I think Manectric has a pretty cool place in the soon-to-be XY OU, especially considering he's an offensive check to Talonflame. +1 Acrobatics does only ~35% after Manectric switches in (assuming it already got Intimidated), so most will want to switch out. This is where base 135 Volt Switches can really hurt.
I'm not really sure that HP Grass is the best choice for this set. I know the metagame is still new, but HP Ice has been really good to me. So many Pokemon can otherwise come in and wall Manectric, which stops its Volt Switching reign. HP Ice may not do anything to Goodra, but it lands OHKOs on Landorus-T, Gliscor, Dragonite after SR, and offensive Garchomp. I use HP Ice way too much to even consider replacing it.
Here's the set I'm using (ok well it's basically just standard...)
(M) @ Manectite
Trait: Lightningrod -> Intimidate
EVs: 32 HP / 4 Def / 252 SAtk / 220 Spd
Timid Nature (+Spd, -Atk)
- Volt Switch
- Thunderbolt
- Overheat
- Hidden Power [Ice]
For now, I'm using these Speed EVs (395 when Megavolved) because they allow it to outspeed up to Jolteon/Aerodactyl/Scarf Politoed/+1 Gyarados and retain a bit of bulk, even with HP Ice's 30 Spe IVs, but it may change in the future depending on how the metagame changes. The speed also brings it to 330 when it's not Megavolved, so it outspeeds +Spe base 100s like Jirachi. I have 4 Def EVs so Genesect gets a SAtk boost, simply because most of them will U-turn out anyway and I want it to be a -1 Atk U-turn. Overheat over Flamethrower because it OHKOs Excadrill and Ferrothorn. Other than that, the set is self-explanatory. I absolutely love Mega Manectric, and I've found a lot of success with it. It checks all those Talonflames plaguing OU and even lives a +3 Aegislash Shadow Sneak (assuming I switch in on a second Swords Dance). The most successful team I found it on was with a Calm Mind/Wish + Baton Pass Sylveon (Manectric takes those Steel attacks that threaten Sylveon, like Scizor's BPunch) and a CB Genesect to make a Volt-Turn core. It's swept a handful of teams at just +1 SAtk. Base 135 is truly amazing. Mega Manectric is definitely one of the more under-appreciated Mega Evolutions.