Does a deficiency in the Sunnah acts of prayer fall under the threat of punishment, and if so, what is the difference between the Sunnah acts of prayer and its obligatory acts?

Scholars differ on whether devotion (khushu') is obligatory or recommended. According to the view that it is recommended, there is no sin in neglecting it. However, according to the view that it is obligatory, one who neglects it is sinful and falls under the threat of punishment.