I wondered when someone would submit something like this. It definitely has its uses, but I don't think you should make "marginal speed gain" a selling point (and considering the presence of cache and .evaluate, I'm actually skeptical of that claim in the first place, not that it matters either way in this kind of system).
As multiple triggers mean at least multiple Evaluate calls, I wouldn't be so skeptical.
I don't like the blizz event response-like madness or that it has to save all event responses everytime, even though most of them are not needed or are empty.
If the syntax for using event responses will be the ugly blizz-like one, I'd prefer to just use blizz' event responses rather than have to use those new hard to remember event responses that also consume extra time during events, I mean really, at least it would make it easier to port old spells to this.
Not a fan of making it SpellEvent_RegisterEventResponse instead of just RegisterEventResponse, it is a little large, and public stuff are already non-scoped, but oh well.