Now that Valve has allowed spies to use enemie's teleports while disguised, there is great oppurtunity to killing clusters of buildings.
First, make sure the engineer is on his teleporter exit, then teleport. This will kill the engineer and not remove your disguise.
Then sap away.
I managed to pull it off once, and it was awsome. Just gotta always make sure to sap the exit so that you dont end up like the engi you tele-fragged.