Editors do know that most writers simsub, but it does make one heck of a difference when editors don't want them.
Editors talk to each other, editors move around, and a writer who simsubs against the editor's wishes eventually gets to be known. Rejections follow.
If all parties say simsubbing is all right, then you can do it. It usually won't help, but you can do it.
But if editors say no simsubbing, and you do it anyway, that is, to be polite, incredibly dumb. Simsubbing is why very few book publishers accept manuscripts from writers these days, and it's why more and more good magazines are going the same route.
The only good thing to say about simsubbing against an editor's wishes is that it really helps writers who don't.
Believe me, word does get around eventually, and few things tick off editors more than writers who do things like this.