If a car with an X code is a true RS 350, it would be an X11.
http://www.camaros.org/numbers.shtml#XnnCodes
As far as value, I think it would all matter who the buyer is/was. Some people start to question everything on a car if they see one thing changed. That would apply to someone who looks closely at "numbers" and wants a car to "be what it is supposed to be"... Other people are more into looks and pro-touring. They could care less about numbers and originality. So, one can usually find the "right" buyer if they are patient enough...
Personally, I would have just waited until I found the car I really wanted. Man, I am still drooling over a '68 Z/28 RS in Sequoia Green that I saw for sale. I think it can still be had (no longer listed). But was still very depressed when I didn't see the listing anymore even though there is no way I can afford it... lol
In the end, it's your car and you can do whatever you feel like doing....