Boomer is great!
...but here's my usual list of nitpicks: in many situations it's ambiguous how you resolve the ability. For example, it doesn't work in Other Worlds, and it's unclear what you do if there are a lot of monsters equally near to you. It also clashes a bit with some special monster movement abilities, and it will move monsters that are ordinarily immobile. So how about this version instead:
"You when fail an Evade or Horror check against Boomer, a monster surge occurs."
That's not quite the same effect but it would be in the same spirit I think. You could make the Evade/Horror checks -2 instead of -1, just to make things interesting.
Siren is fine, except that it doesn't give you any way of remembering that your maximum sanity is one point lower than the number on your card. One of the Golden Rules of AH is that there's always a card or a marker on the table for any on-going effect (except for that stupid Dunwich Black Man encounter thing, for some reason). You could try something like:
"If you fail a Horror check against Siren, your maximum Sanity is reduced by 1. Place a Doom token on your investigator sheet to indicate this."
That's a bit flimsy but at least there's something there. You could use the monster marker itself as the indicator of the reduced sanity, but then it can't be that AND a trophy, and also there's probably not enough space on the marker to explain. That idea does suggest a variation on the ability, though: "While you have this monster marker as a trophy, your maximum sanity is reduced by 1." That's an interesting dilemma.