I'm not too sure the mpg wouldn't drop like a stone after you start modding the 4. At least that was my experience with turbo cars from the 80s.
The $7000 and weight savings with the 4 may not really be valid either. When I bought mine in 08 people were saying what a value the V6 was price and weight-wise but the comparison was apples to oranges. You got A LOT tougher parts such as the trans
and diff plus more standard equipment with the GT, starting with the bigger wheels. The actual difference was minuscule price-wise and I think the 4.6 3v fulled dressed engine weighed a whopping 20lbs more than the iron 4.0. And whether you use it or not, a 3 year warranty (actually 5 on the powertrain) has monetary value. I know all this because I bought the V6 and put a Procharger on it at 14k miles.
Now I don't know what the specific differences are between the 2 as of right now and it really doesn't matter. This time around I'm getting a base GT and putting a supercharger system on that as well.