Tosafos in Chagiga 7a - ד"ה והבכורים - states that only items mentioned in the Mishna we say daily - these are the items that have no measure - are those items that doing more is a bigger Mitzva.
והבכורים אין לו שיעור לא למעלה ולא למטה. הדר פריך בירושלמי רבי ברכיה בעי למה לא תנינן עפר לסוטה אפר פרה רוק יבמה דם צפור של מצורע א''ר יוסי לא אייתינן במתני' אלא דברים שהוא מוסיף עליהן ויש בעשייתן מצוה ואלו אע''פ שמוסיף עליהן אין בעשייתן מצוה:
For other Mitzvos one gets no extra points for increasing them. E.g. if the Yevama spits more, she has not done a bigger & better Mitzva.
If you look in Shulchan Aruch Yoreh Deah קפ"א you will see that the peyos area ends at the bottom of the ear (if not higher).
The בית לחם יהודה adds that one should ensure that they do not grow into the beard area - which starts below the Payos area. He adds that the Ari z"l would trim his peyos every Friday, IIRC.
Since there's clearly no Mitzva-aspect to longer payos, (and the Ari z"l et. al. would say there's a problem with it, actually) it's hard to explain why beautifying the long payos would be a Mitzva.
That leaves us with Rashi.