I've been repairing (mostly) vintage watches for nearly 15 years now, although I hesitate to call myself a "watch maker." I occasionally service high-end watches (Rolex, Patek Philippe, Vacheron, etc) and won't claim to be an expert on that segment of the market. However, I would not consider them to have investment potential based solely on the precious metal content of their cases. The real worth of such pieces is in the movements which, in many instances might cost far more than the case (even one of 18K gold) to replace.
If I was going to buy a watch as an investment based on a high ratio of case value to movement value, I would concentrate on vintage watches - the older the better. In the late 19th and early 20th Centuries, for example, a heavy 14K or 18K gold dress hunting case was often paired with a relatively low cost movement. This pattern continued for decades - until the post-quartz recovery of the luxury watch market. I can't tell you how many gold watches I've seen with crap movements from the 1960s through early 1980s. The problem now of course, is that everyone is scrap happy, so the older stuff is (sadly) disappearing very quickly.
That's my take on it, FWIW, which ain't much!
Matt B.