Knowing what works for Castle MM may not help overvolt a Havoc. I'm not even sure how you would disable the esc BEC which is the 1st necessary step in doing this with the MM - its the red wire in the plugin going to the rx, you pull it out of the lead and tape it off or something to insulate it.

Its hard to believe the limiting 2S to 3S in a Havoc would be the bec.