Lets be real, the Emira is basically a prettier Evora with better tech and interior. Its very similar to the Evora and that's why I think it will hurt the Evora's value once the Emira begins to be purchasable at dealers. Even though in the long run the Evora may be more rare than the Emira, it doesn't offer a ton over the Emira. The only thing it really offers is 2+2, maybe its slightly lighter, and if you are talking 10 or 15 years down the road you may be able to still update the radio head unit to something with more modern tech. The Emira is most likely stuck with the tech in it. Other than that from what we know the Emira will most likely be superior, especially after it matures a bit.
If you are an Evora owner and want top dollar, now is probably the time to sell. The looming Emira will eventually begin to hurt values. Sure, the Evora will eventually bottom out due to its rarity. Its unlikely you will see them, especially the 4XX and GTs for under $30k anytime soon, but its just too similar to the Emira to not to take a hit. I am unsure how collectible it will even be, maybe if Lotus as a brand really does gain the a lot of cache over the next decade or two it will make it more collectable. The Elise and Exige are their own special thing that there isn't really a modern replacement for, so those values will probably continue to climb.
Personally I would be weary of buying a new Evora GT unless it had a heavy discount. If its truly is your dream car and the Emira doesn't appeal to you much then it go for it. Either way you should never buy a car banking on it appreciating or only depreciating only a certain amount. I would always go into a car purchase expecting normal depreciation unless proven otherwise.