You are missing the point. Mewtwo and Darkrai will always give Genesect a SpA boost unless for some reason they choose to give it an Atk boost (aka investing in SpD). Sure you can Bug Buzz for the KO on Mewtwo and Darkrai and successfully revenge kill them, but you are looking at this scenario in a vacuum. If Terrakion revenge kills these two Pokemon with Stone Edge/Close Combat, a lot less things have free reign to set up. Of course, with a Scarf Pokemon you will always have these risks. +1 Scarfed Bug Buzz just begs for tons of Pokemon to have their way (most notably Ghost Arceus, Rayquaza, sub Ho-Oh, etc) with Genesect. I'd honestly choose to U-turn even if it can't manage the OHKO Darkrai/Mewtwo if the opponent has something like the aforementioned sweepers.
And yeah Genesect is 100% getting the attack boost vs Latios and Latias. Poor Latias first no Soul Dew then Ferrothorn then when she finally gets her Soul Dew Genesect is released short after ;_;.
I can see myself using Explosion on something like ExtremeKiller trying to set up but other than that I'd probably prefer a coverage move.
As a side note, I think a bulky Scarf set may be worth looking into. All you need 24 Speed EVs with a +Spe nature to outspeed Mewtwo, allowing for considerable investment in bulk (or Attack I guess?). It does suck that you are basically required to run a -Def or -SpD nature though. It may not have the best defensive stats, but this allows it to abuse the resistances granted by its Steel typing to the max. You do miss out on revenging Pokemon like DD Rayquaza though, so it may not be worth it if you need your revenge killer to do that.
I DONT DESERVE YOU