Im saying ALOT of his heartless killing ability comes from the keyblade.
Well, Sora could kill Shadows and Darkside perfectly fine with Dream Sword/Shield/Staff as well. Hell, Sora managed to kill Shadows by making them roll off of him in KH2.

But I guess you're right to say that the Keyblade does help with it, since it's more "super-effective" against Heartless than other weapons.

Argueing that he is stronger than Squall or Cloud because they cant kill heartless like Sora can is invalid.
But Cloud and Squall can't kill 1000 Heartless single handed.
I guess we can presume destroying a heartless with any other weapon merely results in its temporary return to darkness or something of the sort.
Correct. Killing a Heartless with any non-Keyblade weapon causes it to return to the Realm of Darkness, which eventually will cause it to return once it recovers from the damage.