Where is the only Aerostar 800 ever built?
According to this page the airframe was donated to Embry-Riddle Aeronautical University:


  The Superstar airframe was later used as the prototype Aerostar 800. The prototype 800 was the Superstar airframe with two more seats (due to a 32” extension of the cabin) and featured two 400-hp. Lycoming engines. The 800 was under development when Ted unexpectedly passed on in 1976. Piper continued development of the airplane as a replacement for their P Navajo but never certified it and eventually donated the airframe to Emery Riddle [sic] Aeronautical University. 


This is the only reference I can find to the ultimate location of the Aerostar 800.