So i've been using Sableye fairly extensively this gen too, and i have to say that i switched to Knock Off and never looked back. Usually, when you change moves like that, there are times when you go 'man, having that would have saved my ass' or 'i wish i still had that'. I can't remember having that feeling, like, at all.
Knock off lets it completely trash Heatran, one of its best counters. Suddenly, it's lost around 20% of its HP and it won't be recovering that back via leftovers. It also lets you get rid of Toxic Orbs, which
A:Rotom-w volt switch to gliscor/breloom is no longer guaranteed activation
B:after knock off, Aromatherapy/Heal Bell gg's the aforementioned pair
It lets you get rid of leftovers, which lets sableye have an impact, even if the other team has Heal Bell or Magic Guard pokemon. With it, Sableye is never truly useless. And of course, getting rid of special attacker's life orbs and choice specs makes it harder for them to break sableye, too.
Anyhow, Knock Off for first slash.