Very nice feedback, thanks!
I can add "in Arkham" to clarify that the Boomer's ability doesn't matter in OW. I don't really mind that it will move monsters that normally couldn't. And every situation in the game where there's a tie, first player decides if everyone can't agree. That's all over the rules. The monster surge variation is actually really cool though. It is certainly in the spirit, although I think I'd rather have both! The Boomer makes the most sense with the ability the way it is, and I don't think the issues you raised are a big deal. So I'll use the monster surge ability for a new monster instead!
It's also kind of funny that you mention the Siren not giving you a way to track your maximum Sanity. Neither does Cthulhu or Mythos Lore, which are probably the two most common ways to have your stats permanently reduced. How do you track those? Do it the same way for the Siren. (Or just use a doom or yellow sign or whatever, as you said.) Again your version will be an awesome idea for another monster though.
You better use these ideas first, or I will! XD