You can measure the efficiency loss with Console.start() / Console.stop(). Be aware that this operation has O(n) complexity, so it will increase for bigger projects.
As a general rule of thumb I would suggest to keep a reference around for events that are often triggered (or if they are in the MIDI callback then it's out of the question to call the method obviously), but if it's being used just when the user presses a button, the additional 2-3 milliseconds it will take won't be noticeable.