There are all kinds of things that can happen and it depends on a lot of different factors. Match (a member who I haven’t seen in a while) did a ton of work and made some great charts. Sometimes you simply kill the emitters right off the bat. Other times it will stress them and cause an early grave. Other times you will actually see less lumens. If done correctly with enough heat transfer you will often get an increase without damage. Almost all of us modders here are overdriving our emitters well past the factory specs.
Here is a link to some of Match’s great work.