The benefit of having all spell event responses available on all spell events is significant, the speed gained by removing it is not.
I could write another paragraph on why the feature shouldn't be optional, but since the speed gain is not significant in the first place, I don't see a reason to spend more time explaining this.
Besides, the bj_wantDestroyGroup-esque syntax is ugly.
If your particular map doesn't require this feature then you can always delete the event responses code from the library yourself, nobody is forcing you to only use the code as provided.
The second optimization you suggested is meaningless, an if statement isn't faster than an exitwhen statement in any meaningful way.