Remember, the Mustang is a muscle car. It is designed to be heavy, seat four adults, go straight, and be fun to tear up the streets. That is why torque is so important.
I might disagree with this assertion, that the Mustang is a muscle car, especially such a caricatured description of one above ("designed to be heavy???? It's a car, not a tank). Ironically, the Mustang was originally designed much in contrast to the muscle cars of the day, emphasizing a smaller trimmer size, smaller displacement hi-po motors and a far more balanced approach to performance than some crude, stop light, straight-line beheamoth. This quite distinctive approach to performance might better be called a Pony Car. Granted some later big-block Mustangs did more closely approach the muscle car idiom of a big car with a bigger motor, the 390/428 Mach Is being a good example example. Most Mustangs have been much closer in character to the original Pony car idiom.
That all said, the 4.0, a fairly old design now, makes decent if not overly impressive power. Yes, the size will give nice off the line oomph by dint of low rpm torque, but it will likely peter out rather quickly, especially as compared to the more advanced motors now out. Ultimately performance does depend on horsepower, which is the measure of the actual energy output of a motor. I'm not sure how much satisfaction a V6 Mustang (202hp / 235 lb-ft) owner might get when a 3.5 Nissan Altima (250hp @ 5,800 / 246 lb-ft @ 4,400) goes ripping past his/her doors.
I do suspect that the Duratec 3.5 will find its way into the Stang in one to two years and will likely mirror the power outputs of other modern V6 designs such as Nissan's and Honda's, with a commensurate increase in performance, both quantitative (faster numbers) and qualitative (much broader spread of power that will sustain even at high rpms). It may even be a touch lighted due to its all AL construction, lending additional benefits in terms of improved handling, where lighter is better.
As for 60 vs 90 degrees, the former is generally a much better layout in terms of balance and NVH. The reason for the latter is generally a cost saving motive in that such a block is adapted from an existing V8 design and can use much of the same tooling during the manufacturing process. NVH can be controlled by dint of split journals on the crankshaft and the addition of balance shafts to counteract remaining imbalances, but that's a lot of extra, non-power-producing weight.