Do the Theravādins believe in the Buddhaguṇas? In the Pāli Theravāda tradition, are there any specific guṇas that a sammāsambuddha has that an arhat has not?
According to the sutta below, the main difference between the Sammasambuddha and the Arhat is that a Sammasambuddha discovers the previously unknown (or forgotten) path, becomes an expert of the path and is able to explain and teach the path to others.

On the other hand, the monk freed by wisdom (and therefore an Arhat) does so, following the path taught by the Sammasambuddha.

The other qualities described in the suttas like psychic powers (e.g. Moggallana) or the ability to be an eloquent and articulate teacher (e.g. Sariputta), may also be obtained by an Arhat.

Please note that when I say Arhat above, I mean a non-Buddha Arhat. This distinction is important, because the Buddha is also an Arhat.

From SN 22.58:


  “Mendicants, a Realized One, a perfected one, a fully awakened Buddha
  is freed by not grasping, due to disillusionment, dispassion, and
  cessation regarding form. They’re called a fully awakened Buddha. A
  mendicant freed by wisdom is also freed by not grasping, due to
  disillusionment, dispassion, and cessation regarding form. They’re
  called a mendicant freed by wisdom.
  
  A Realized One, a perfected one, a fully awakened Buddha is freed by
  not grasping, due to disillusionment, dispassion, and cessation
  regarding feeling …
  
  perception …
  
  choices …
  
  consciousness. They’re called a fully awakened Buddha. A mendicant
  freed by wisdom is also freed by not grasping, due to disillusionment,
  dispassion, and cessation regarding consciousness. They’re called a
  mendicant freed by wisdom.
  
  What, then, is the difference between a Realized One, a perfected one,
  a fully awakened Buddha, and a mendicant freed by wisdom?”
  
  “Our teachings are rooted in the Buddha. He is our guide and our
  refuge. Sir, may the Buddha himself please clarify the meaning of
  this. The mendicants will listen and remember it.” 
  
  “Well then, mendicants, listen and pay close attention, I will speak.”
  
  “Yes, sir,” they replied. The Buddha said this:
  
  “A Realized One, a perfected one, a fully awakened Buddha gives rise
  to the unarisen path, gives birth to the unborn path, and explains the
  unexplained path. They know the path, understand the path, and are
  experts in the path. And now the disciples live following the path;
  they acquire it later.
  
  This is the difference between a Realized One, a perfected one, a
  fully awakened Buddha, and a mendicant freed by wisdom.”