I live in Buffalo and have driven a GT in the winters here for the past 4 years or so. I have a dedicated set of snow tires on a pair of steel 16" rims. I have never gotten stuck, and frequently pass Jeeps and other 4WD vehicles stuck in a ditch.
It takes time to develop the feel for it but overall my car is much more maneuverable in the snow due to the inherent oversteer characteristics of RWD (which in the snow i think is much preferable to an understeer situation you get with FWD).
Also, one key point to keep in mind is that the narrower the tire, the better it will handle in the snow. Most of those who claim that FWD is better is because they generally have narrower tires than what you would find on most RWD vehicles. If you have equivalent width tires on a FWD and RWD vehicle in the snow, the RWD would typically outhandle the other just as it would in a normal pavement situation. The only difference would be that their oversteer and understeer characteristics would be enhanced significantly due to the reduced traction on the drive wheels.
By "more maneuverable" do you mean in executing fishtails? LOL
I have been stuck behind more than a few rear wheel drive cars on hills around here waiting for them to park the damn thing so I can get around them in my FWD or 4WD.
If you are "passing 4x4s and Jeeps in ditches" it is because those drivers were driving way too fast for conditions and/or have no idea how to drive in snow even with 4WD, thats all.
Has nothing to do with your rwd being better in snow.
I have driven lots of rear wheel drives and lots of FWDs over the years in snow.
FWD wins hands down and it is not only due to tire width (which DOES factor....narrower the tire the better bite thru the snow you get versus floating on top of it).
The weight distribution of a FWD car - more weight over the drive wheels - will have it winning hands down in comparable testing. More weight over the drive wheels nets more bite - more traction. And obviously traction is what keeps you going.
You can load up the butt end of a RWD with sand bags but then that introduces other handling issues.
Oversteer is definitely NOT your friend in slippery conditions. A very small steering input can result un a HUGE change or complete loss of traction in a RWD car in a slippery condition. Steering with the drive wheels in a FWD means that you are pulling the car in the direction the wheels are pointed (to a point at least)...in a RWD, you can turn the front wheels but the drive wheels are still pushing straight ahead and you can "snowplow" when its slippery.
On ice, all bets are off....even 4WD sucks on ice.
The question from the OP was if a 2006-08 Mustang GT was "safe" in snow.
I think the answer has already been given - in the hands of an experienced driver, perhaps it is doable, depending on how much snow and the terrain we are talking about. In the hands of a youth, more likely to be a real problem than not.
No offense to the younger crowd here.....I know I would not have my young son/daughter driving a rear wheel drive sports car with a 300 HP engine at ALL unless I was very sure of their maturity and skill level...and even then, NOT in the snow.
Just sayin'.