The 928 was never a car people longed to own, so I think the investment potential is pretty minimal. However, the 90's GTS is probably getting close to the bottom of it's depreciation curve, but I doubt you'd ever get anyone to pay more than the $50k or so it takes to get into a really nice one now.
Manuals are rare and probably the most desirable from a collector standpoint.
Cars as investments are rarely a winning proposition. After you factor in what it costs to maintain, store, insure, etc. a car for x years I doubt you'll be making money. Every now and then there's a big surge in the collector market, but that's usually confined to muscle and pre-war cars. Those cars you see selling for a half-mil on Barret-Jackson usually have more than that in the restoration or are near zero mile 1-of-few cars that got mothballed when new.