I have a very good friend who has been in the appraisal business for over 35 years. He has been in the car hobby his entire life. He is now 66 years old. He owns 7 Pontiacs and 7 Corvettes. He also sits on several boards for the car hobby as well as the appraisal business and the collector car ins. co. industry.
His opinion on adding OEM optional equipment to classic cars is this. IF you add OEM optional equipment using CORRECT OEM and or NOS and or original parts, YOU WILL NOT detract from the car's value in any way as long as these are bolt on type options (which most usually are anyway). In fact, rounding up these optional parts and adding them to the car will often make the car much more desirable to many potential buyers/collectors down the road.
On the flip side, aftermarket add-ons or hacking a car up will only detract from the value of a car. Adding vintage air is yet another thing all together because it falls somewhere in between. Although Vintage Air is often consider a bolt on item, it is not. It does require a fair amount of cutting/hacking and or altering of several areas/parts on a car. Thus in most collectors opinions will detracts from a car's value if it's something special to begin with. For example a true 400/4-speed car all matching numbers car might not be a good candidate for Vintage Air, whereas a car that was originally a 350 car and is now a 400 car would be the perfect candidate for Vintage Air conversion etc.
However, that same original numbers matching 400 car would be a great candidate for a fold down rear seat, power trunk release etc. All true bolt on items that will certainly add to the car's value and most definitely coolness factor!!!
In fact, the all original 400/4-speed convert that I am working on now we will be switching from standard interior to deluxe interior as well as adding SEVERAL OEM optional equipment to this car and we expect only increase it's value. However, we will be VERY critical in doing so as we will be using NOS/OEM parts and all the work will be done at a very high level, no butchering/hacking or "rigging"...
