In my experience premades generally wont split and will be more interested in winning through kills rather than objectives. I know its already been said but you can exploit that on maps that allow it by splitting into smaller groups. The premade will always gravitate towards the team thats more densely grouped and leave you alone.

Recognizing them early and 2v1ing them with the other team, before they battle high/fever out of control seems like it *should* work, but never does. I could speculate on a lot of reasons why it doesn't.

Reality is that they're going to be more organized and skilled than randoms every time, so they're almost always going to win. And that's really the way it should be.