No, as the error suggests you need some response descriptor defined. It doesn't need to be complex (it can map a single data item like a status flag into an NSDictionary).
Don't worry about performance until you have reason to (profiling shows a problem).
That said, the most efficient way for RestKit to operate (at runtime) is to not have multiple response descriptors to search through, so be as specific as you can with path patterns and keypaths.