I'm pretty sure 16 inch wheels will not work. The spacing between wheel and brake caliper was pretty small when I saw it in person. And I don't think there was nearly enough space.
If the rotors are the same size as the current GT's, you can use as small as 15's. If the rotors are 13", you have to use 17's.
I thought I read they changed the bolt pattern on the '05 wheels, if that's the case then they won't fit.
At a certain point you have to break with the past if you really want to improve the car. Yea it would have been nice if the old wheels still fit, but times change and cars do too.
The backspacing was changed for a reason. The larger back spacing allows the suspension arms to be longer which improves both ride and handling.