Does this spells the end of leechers?

Maybe if you hit double what the monsters HP is you get double the monsters exp? Doesn't sound like it would work, i mean, imagine how much exp you would get using anything that hits over 100k on a snail.

You're making a common mistake - Overkill is irrelevant. You can't get credit for taking more HP than a mob has. If it has 100 hp and you do 5 billion damage to it, it's still only lost 100.

The existing formulas already use this concept.