I remember your criteria.
Us market car right? (The GTO is made in Aussy land, but since it's a "Pontiac" we will ignore it's Holden roots.) They are WAY overpriced, but an excellent car with in a body that does not do anything for me. (And the rest of the US market from what I can see.) If she wants brute power, refinement and a nice all around coupe that is "Domestic" the GTO is a good place to look.
The 4 door charger is a family car. Fine if she wants that, but going from a two door coupe to a 4 door sedan of twice the size generally is not my idea of an upgrade. Power is there, refinement in the suspension is there, but the car is just FUGLY in my opinion.
If she can live with driving another Japanese car, the G35 is a great vehicle in any form. (Even the 4 door is sporty looking.) The coupe however is very nice, well built, great ride and handling and plenty of A/M support with more growing every day. The VQ V6 is a great motor too. (My personal pick if I had to replace the Supra with a "NEW" car on a budget, and wanted to stay rear wheel drive etc.)
You said no EVO/STI etc. (The SRTNeon is not an option, they are fast, but FUGLY and the build quality screams "I'm as cheap and shitty as I possibly could be, but like every other ugly date, I put out.")
You said no Audi's or BMW's. (VW's either.)
Too bad, the A4 line would be perfect for this gal. There are S4's with V8's in them, AWD and great build quality that she would love to drive for many thousands of miles to come. (nothing like a 300+hp AWD car based on a well built chassis with a high quality interior...)
You guys living in Canada should consider all the AWD options she has.
Subaru WRX's and Legacy's are all nice vehicles, but they are 4 pots, and she "hates" that sound. (Have her drive a STI, and then say that 10 times fast... LOL Sound v/s power, I'll take the power please.) The STI does sound like a pissed off sewing matchine, but it's really the people your racing that notice since they are looking at the BACK of your STI so they can hear the exhaust as you blow their doors off. (Nothing a nice Ti catback, and few mid/DP's can't cure on the STI or EVO for that matter.) They are wicked fast, and yes, those vents, scoops and wings are functional, but butt ugly in some cases.
Brings us to the Mustang, the car she wants. If she buys a GT with the 300hp V8, it's going to be the best deal cost wise, and she ends up with a instant "classic" new car that she will see herself driving in all over town. (Clones of this car will be everywhere.) Avoid the V6, I've driven this car as a rental, and the V6 is just loud, not fast. (Sounds fast, but every 4cyc turbo car on the planet will be kicking your ass, and any well done Supra Turbo will be handing it to the V8 powered ones as well. Look out for the new Cobra, it's getting a 475hp/tq supercharged V8, and will be pretty fast right out of the box from what I can tell. It's the 03 Cobra all over again.)
So, sorry about the long post Grim, but I have alot to say. (And as you know, if you have anything worth saying, you should take the time to say it well.)