Many hadiths emphasize on Fajr Sunnah. Therefore, according to Hanafi School, if someone reaches to Masjid at a time when Jammat has been started, and thinks that if he at least can find one Rikkat of Jamma'at, after performing Sunnah then he must offer Sunnah first. Therefore for Takbeer-e-Oola, leaving Fajr Sunnah is not correct.
ALLAH Almighty knows best
DARULIFTA AL-IKHLAS, Karachi