Mega Manectric is going to be really solid. 135 Speed outspeeds Ash Greninja which is nice, and 135 SpA is solid too. For example, MegaMan OHKOs Offensive Lando-T (which is not often Scarfed anymore, or even Double Dance which IMO is a bit slept on) with HP Ice 100% of the time, something Tapu Koko needs a Life Orb to do (and Magnet/HP Restore Berry is much better than LO). In addition, access to strong Fire-type coverage is another important niche MegaMan has over Koko, letting the dog blow past Ferrothorn/Magearna/Scizor and more. Mega Manectric is also a good offensive check to Mega Pinsir due to Electric-typing and Intimidate, plus the fact that MegaMan outspeeds and OHKOs with Overheat/Thunderbolt/Volt Switch (the last has a 62.5% to OHKO from full, so Rocky Helmet damage from Lando-T is enough).
252 SpA Manectric-Mega Thunderbolt vs. 252 HP / 252+ SpD Celesteela in Electric Terrain: 326-386 (81.9 - 96.9%) -- 62.5% chance to OHKO after Stealth Rock
252 SpA Manectric-Mega Thunderbolt vs. 0 HP / 4 SpD Heatran in Electric Terrain: 217-256 (67.1 - 79.2%) -- guaranteed 2HKO
252 SpA Manectric-Mega Thunderbolt vs. 248 HP / 0 SpD Zapdos in Electric Terrain: 249-294 (65 - 76.7%) -- guaranteed 2HKO after Leftovers recovery
Also, Overheat>Flamethrower, since it OHKOs Ferrothorn 100% of the time and does more to AV Tangrowth (which usually comes in as a check). Plus, you'll likely just end up Volt Switching out, and since you outspeed the SpA drop doesn't matter.
252 SpA Manectric-Mega Flamethrower vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 146-174 (36.1 - 43%) -- guaranteed 3HKO
252 SpA Manectric-Mega Overheat vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 212-250 (52.4 - 61.8%) -- guaranteed 2HKO
252 SpA Manectric-Mega Overheat vs. 252 HP / 4 SpD Mawile-Mega: 304-358 (100 - 117.7%) -- guaranteed OHKO
Potential/likely core: Mega Manectric also forms a great VoltTurn core with Tapu Koko, and the added boost to Volt Switch and Thunderbolt really is nice for Mega Manectric, so expect to see this core fairly often, along with good old SR Lando-T as a Ground immunity/Hazard setter/and Intimidate U-turner.