50 MPG should be attainable in your Yaris given that it has the same engine as my Echo and a somewhat similar body style. Adding a grille block should bring you closer to the goal.
Keep up the good work ! You have done the hardest part which is adjusting and changing your driving habits. Aero mods can help squeeze a few more miles out of a gallon of gas.
50 MPG was my objective a year ago. I am now aiming for 55 MPG.