Today, I read an interesting article relating to exotic/supercars values with a BMW M1 as the feature subject. The car, like the Callaway Corvette was done in "conjunction" w/ outside sources (body, etc) and in limited numbers (about 458 cars made over a 4 year span 78-81). Those cars have a strong following and the market appreciates their heritage...
There are other cars in limited numbers out there but, I thought this one was close in relevancy 
thoughts???
I think the M1 may be in a slightly different category. Consider that this was a completely new vehicle, sharing nothing except the block, brakes, and instrument cluster with other Bimmers of the time. It was mid engined, exotic, and had a prevelant racing career (Not that Callaway does not participate in GT racing).
It is hard to accurately compare our potential collector cars today with those of yesterday, but I feel that Callaways will most resemble other rare engine optioned muscle cars such as L88's, or Hemi Mopars. AeroBody cars will be in a different league, which has no history to compare, except for Motion Phase III Vettes. The awareness of the potential values of current specialty cars is at an all time high, as everybody learned the lessons of 4 million dollar Cudas, so people are looking for cars to keep in the hopes of scoring in the future. I can't count how many GNX's, Turbo TransAms, Anniversary Vettes, Vipers, Ford GTs or Cobras are all kept sitting with delivery milage, waiting to emerge from their time capsules 30 years later. I am guilty of this with 4 Callaway C8's. But the old muscle cars were different. Nobody (few people) thought that they would have the future values, because people did not order rare cars to make them rare. They are rare because after the fact, it turned out not many people ordered them because of price, performance, or obscurity. Plus, there was no real history of sky rocketing values for anyone to try to emulate. So if you were lucky to pick up a Hemi Cuda in 1978, you beat it like a rented mule. My father bought my cousin a 69 Shelby GT500 in '77. He was in college. He totaled it. We pulled the motor and scrapped the car. Who knew? Keep in mind, that most of those cars were destroyed due to several factors:
1. There was very little media coverage of specialty cars in the 60's - 70's. Chances are, if you didn't live in Cannondale, you didn't know what a Yenko was. If you were not in Grand Rapids, chances are you never knew what a ZL1 was. Consequently, cars were bought and sold, and as the car passed from owner to owner, the story faded slightly (unless you were in the know). Ignorance is the biggest enemy of any valuble car. Just ask the guy that I got one of my C8s from. He removed the RunFlats from it, and sold them for $350.
2. Future values were less of a consideration than performance. Back then, you bought a ZL1, L88, or COPO and beat the snot out of it. You took it apart, put Thorley headers on it, cut the 1/4's for slicks, and cut the pans for ladder bars. If you bent it, you scrapped it. This makes clean examples super desireable.
3. The main difference between your L89 Vette and any other Vette of the day was the engine. Many hot engines made the trip from engine bay to engine bay. This is why numbers matching cars are so much more desireable. Once the "valuable" engine was gone, that original L89 Vette, was just a shell, not a valuable VIN or option tag. You scrapped it or sold it.
We know better today. We will cherish our Callaways and other rare, fast cars. We certainly will not pull a TT motor out and fit it in a C5 or other newer C4. Therfore, there will probably be a higher percentage of surviving cars today, as opposed to the 60's and 70's. And it is highly unlikely that any car, short of a Shelby Omni, will fall below the radar, only to emerge years later as a sleeper. Those muscle cars did. I bought a 69 Yenko Camaro without knowing what it was!!
The great positive for our cars, Callaways especially, is that they were born in a time of performance impotence. 385-400 hp sure stuck out in 1988. That was in the league of the world's finest offerings.
Another positive is Callaway's racing heritage. This adds credibility to any program, such as Shelby or Cooper (big in Europe).
The biggest value holder for Our Callaways is that bow tie on the front. When a few more years go by, and all of the Chevy boys need to fill their garages with the most rare performance cars of the late 80's and early 90's, they will naturally gravitate to the TT's.
So after all of this rambling, I fell comfortable that Callaways will be at the top echilon of value, similar to Hemis, Shelbys, and COPOs today. Keep in mind, that while there are over 500 TTs built, it was over a time frame of 5 calendar years. I think about 188 cars was the most in one year. That is right there with 69 Yenkos (201), 71 Hemi Cudas (about 140), and other cars that bring 6 and 7 figures these days.
I hope this helps.
Anthony
