-Sound works, occurs when Boomer explodes... but that leads me to the cons:
-Sound timing is off-> the BOOM part of the phrase happens after the boomer explodes
-Sound quality isn't terrific, but it's audible
-I'm not even slightly familiar with how modding works, but this just my opinion as a player using the mod: I can't see this as being something that's easy to time. In order for the 'BOOM' to happen right as the boomer explodes, the 'WTF' would have to happen before the weapon/bullet hits the boomer. And even so, with all the other noises (i.e. gunfire, common infected screams, explosion noises, general ambience, etc.) probably going on, it'd be hard to hear the wtfboom sound, anyway.