MongoDb - Is $where more performant when using functions stored in db.system.js?

MongoDb recommends that "In general, you should use $where only when you can’t express your query using another operator" due to perframce reasons. However, it appears that we can store Javascript functions server-side using the special table system.js.

So instead of doing this:

db.myCollection.find( { $where: function() { return this.credits == this.debits; } } );

Do this instead:

db.system.js.save({ _id : 'queryFunction` , value : function() { return this.credits == this.debits; } });
...
db.myCollection.find( { $where: 'queryFunction' ); //not sure about syntax

Would the second variant reap any performance benefits; and if so, are they significant?

References:

• $where
• db.system.js