Value is in the eye of the beholder. Vehicles are typically never a wise investment option (neither is the stock market as of late
) Especially our Porsche's that need/want to be driven and I think that's part of the appeal. There will always be some inherent value to a prospective buyer if you modify or keep original but near originality has some benefits now that we are approaching 40yrs+ for these cars.
If you have an original motor, tranny etc I would personally urge to keep the modifications limited or easily changeable (I fight that urge every day and I've only had my car for 7months or so). But I think some of the fun is that you can make many/little modifications and the car will take on some of your personality.
As for the CoA, it's only $100. Again, if you have an original motor, etc and want to get something official to match...great. I have a matching #'s car and got one and it's only 80% accurate and told me little information I didn't already know but I've wasted $100 on much more useless stuff before.
No matter what, drive it and enjoy it don't let her sit thinking your gaining value by not driving it.