A direct quote from you... I assumed this whole time the Altima had a supercharger, because you said it had one.
The Altimas curb weight is 3417 (for a 2009 3.5SE), and the Mustang is 3270 (actually a 1986 GT was the first one I found). Here we see a 147 lb difference when taking stock specs. The Mustang at 225hp, the Altima at 270hp.
Just grabbing info off the internet real quick, but it appears your number guesses for the cars might be skewed slightly in the Mustangs advantage.