If you look on the forums, s-plan (invoice?) isn't the lowest price if you compare to some of the deals that posters got. But if it's the best deal "you" can get and you don't even have to negotiate for it, why not go in with the s-plan as a starting point? It beats using MSRP as the starting point.
Withholding info isn't going to help as much as you think. Dealerships are used to customers lying and being sneaky- after all, they did write the book on it. "Deals" at the dealership don't come from negotiating skills alone. It takes the right place, right time, right manager for it to happen.
Why do you say that?
Didn't you get a deal similar to the type that you "get a kick out of" other people saying that they got? If you got that kind of offer, what makes you think other people didn't get a good deal also? People do embellish their own personal purchase prices, but you aren't the only one who gets a good deal.