I think it is a mistake to apply the same logic you would use with other cars to the Elise and Exige. Granted, there are the collectors out there but these are designed to be track toys. As such, the limitations of the original design are very well known. Having mods from one of the "codified" vendors mentioned above to solve these problems adds value. So things like baffled oil pans, fuel surge tanks, tie-rod reinforcement, hardened cams etc become bonuses not detractors. Also, things like BOE REV 300/400 would add a fair amount to the value of the car (cars with these mods sell for higher prices than similar cars without). As would significant suspension upgrades. On top of all that, a car that is tracked regularly is generally better cared for mechanically than a street-only car (regardless of whether or not the owner took the car to the dealer for all scheduled maintenance). It's a requirement. Oil, trans-fluid, brake fluid, coolant etc all get changed way more frequently than in a street car. Any maintenance item on a track car becomes a safety issue to the driver. That inspires the driver to fix it correctly. Even a poorly maintained street car will likely not just break and cause you to flip the car or something, so there is less incentive to do maintenance beyond the service manual guidelines.
Now the other side of this coin is the cosmetics and the removal of creature comforts. Most track junkies are not as concerned about cosmetics and more concerned with function (not all, some). So, wings bolted to the rear clam, canards hastily riveted to the front fender area, forgetting to put some type of protection from sand/gravel on the headlights or windshield will detract from value. These are style issues as well. That being said, some cars have great external mods that are subtle and generally make the car look better and that becomes personal preference. Some of the more hard-core will also do things like removing the AC which will add lightness but reduce value for most buyers.
A collector would be more inclined to buy a completely bone stock Lotus with no mods. But they likely wont be driving the car as it was intended. Just saving a piece of automotive history. ANY wear and tear to these people will detract value. I have seen very very few people in this category. The car is just not that old yet. I am sure that one day it will be a collectible but it isn't today and I don't think it will be any day soon. They are just too fun to drive.
Update: To be clear, I am not saying that upgrades are "worth the money", almost no upgrade pays for itself when you sell the car. But you will get SOMETHING back.