I thought I would chime in since I just bought a cayman S. I previously had a 95 993 which I sold to buy the cayman S. The first thing that I would mention is that I made money on the 993 which almost never happens. I purchased my 993 two years ago for 33,000 and sold it for 37000. I sold it to a friend, otherwise I think I could have sold it for even more (the car only had 20000 miles). I almost feel like I was paid to drive the 993.
I am positive a corvette from the same era has depreciated much more.
I am taking the cayman to the track for the first time this weekend (buttonwillow), so I can provide more feedback on monday. I have also driven and ridden along in a C6 Z06 at streets of willow, and I tracked my 993 a few times. I have also done a drivers ed and some auto crosses in my 993 with the porsche club. I am still a beginner when it comes to performance driving so keep that in mind as you read my comparisons of the cars. I will start with the C6 Z06. The Z06 I drove was insanely fast, and had so much torque that it made things easier in certain parts of streets. For example, entering the bowl in the Z06 I just left it in 3rd, and it happily pulled right out of the bowl and quickly hit over 100 heading into the chicane. The power difference is immediately noticeable. Also with all of the rubber it hits corners like it is on rails. It is an impressive car in some respects. I would say the negatives are that you get almost no feedback, things feel very loose, and even this very new car rattled a lot. Also I was hanging out with guys from the corvette club, and their cars broke more than any other group of cars. I saw a balance pulley come loose, all kinds of problems with oil (leaks, over heating, starvation), a cracked hub and a myriad of other problems. All of the corvette guys add extra oil because it is so easy to starve the engine of oil (of course maybe my cayman will have this problem also).So in summary the Z06 is ridiculously fast (both in the straights and corners), poorly built, unreliable, and offers almost no feedback - at least to a beginner.
My 993 by comparison was underpowered, but provided way more feedback, and in my biased view was way more fun to drive. I like the chassis and the feel of the car, but the stock springs (even the m030) are way too soft, and the car rolls around way too much. But the car feels as tight and rattle free as a new car. Also throttle steering is very intuitive. My lap time in my car was faster despite giving up nearly 200 hp. Of course I was taking it easy in the z06 since it is not my car.
I have not really pushed the cayman yet (since it is not quite broken in), but the suspension feels much better than my 993, and the car feels very balanced. I think it will be a much better autocrosser. In fact I can't wait to try it in the march autocross. In the NSS class with my 95 993 C4 I always felt that I had the slowest car in my class, now with the cayman I should be out of excuses.
I think that the cayman should be very competitive in the NSS class. If the boxster is any reference the cayman will probably not hold it's value as well as my 993 did. I think 993 values are so high due to nostalgia. It's almost like being able to buy a classic car that performs like modern cars and has modern conveniences. I think that newer porsches will probably depreciate similar to other cars, but if you want an investment, buy real estate not cars.
FWIW that is my 2 cents.
Mark