Your analysis makes some sense, however, if you're truly concerned with the best value for money you'd be far better off buying a used 911 (of your preferred vintage).

Buying a brand new car is almost never financially savvy (unless it's a Ferrari or maybe GT3RS), so when making such a purchase it's important to make sure you limit your regrets as much as possible (i.e. will you regret not having the extra power of the "S").\

If I were to buy a brand new Porsche, I think the 3 best values in descending order would be GT3 (current one with GT1 motor), Cayman S (pending model with newly available LSD and DI motor), then Carrera, then Carrera S.