FWIW & IMHO:
I don't think that GN'S, or T-Type's, or Turbo-T's, or TTA's have gone down over the last year. I just think that GNX's have escalated more than any of the other Turbo Regals. Because each car is unique to itself, it is very hard to sit back and pass fair judgement over this situation. You virtually have to see each car in person and impartially judge their value by mileage, condition, documentation available, how many owners, and a multitude of other aspects/parameters to reach an unbiased opinion. There is definitely a seperation that is getting larger between what one would consider a museum quality car, and whatever else is runner up condition. More and more people are becoming aware of what consitutes a pristine car, and what isn't. Remember when you heard years back that a lot of collectible vehicles were going over seas? Well, they too, have learned what is really worth money, and what isn't. You don't see any junk going to Japan anymore. Not everyone wants, or can afford a no 1 car sitting in the garage, either, and that plays a part in the collector market.
These cars may not escalate steadily, but year by year they have gone up. I think that they always will. If you really want to see how these cars have done over the last 5 or 6 years, go to Kirban's site and compare the average prices of 1 year to another and I think that the results will speak for themselves.