Jim, you need to chase some of the guys dissecting and polishing various old chips. ?My gut feeling with regards to the BS170 saga is that a well soldered joint with good penetration to the topside offers more thermal protection due to the relative transfer lags than anything done to the plastic case. ?The SOT23 package BS170 might actually be a better option as they have thicker and shorter terminations. ?As I say no science involved in this :)
Best 73, Clive G4KCM
Clive - G4KCM