Of course it hits harder, that was never really up for debate; the question at hand is if that harder hitting is worth giving up the chance to strip foes of their items. I have to say that i fought a couple of heatrans on stall teams, and before i started using Knock Off...that would've been nearly GG, right there. Now? I just Knocked them off and stalled their asses out, then proceeded to win.
So, if you really consider foul play to be better, fine, but list some stuff that it does. is it getting crucial 2hitkoes, or even Ohkoes? What mons does sableye really need it for, this gen?
T.k to make a long story short that set sucks, so badly. You instantly lose to any fire-type, anything with heal bell or natural cure isn't too fussed either. Taunt smashes you too, the other side can set up hazards all over your face. Any defensive pokemon with recovery can just whale away at you, and at best it's just a stally stalemate.
Edit:
It was running from non-mega'd Zard in the first place, and zard can still just blitz it in the face and Sableye can still taunt it. Foul Play is super helpful for zard, yeah, but it's still something you should run from a lot of the time.
As for talonflame, 0 Atk Sableye Knock Off (97.5 BP) vs. 4 HP / 0 Def Talonflame: 109-130 (36.5 - 43.6%) -- guaranteed 3HKO after sandstorm damage, so it does take it pretty hard after rocks and get rid of its leftovers/offensive item. Foul Play does harder, hits more consistently, and OHKOs after rocks, but you're still not defenseless against it.
And more importantly, Sableye is a stallbreaker first and foremost in my opinion; he should be better able to do that.