Thats what they say he is and I think so. Sending a group would have not only been too suspicious but they would have been easily spotted like Naruto and the others were. One person attempting to infiltrate any organization would always have higher chances of success than a group of people would. In any case I don't see Sai standing any chance against either Oro, Sasuke or Kabuto.
Sai freaked out when he heard someone coming in through the door it wasn't because of what he saw in the book.