"accessing missing properties throws" will just lead to a lot of use of try/catch, and the issues that entails. It's frankly doubtful whether it will speed things up at all, if there is any penalty to throwing try/catch everywhere.
That won't be a problem in SoundScript though, because any code which tries to access a property which might be missing is unsound, so it won't "compile" (or rather, it won't pass the type checker).
