I think the R value will be fine for some time to come. The '00 R held its value for a loooong time. (Granted, there were only 300 of them, but still...) The only GT350s that will probably take a big hit are the '15/'16 base and Tech Pack cars.
15s are going to be the top value GT350s, regardless of how they are equipped due to low volume.
The 16 tech pack cars will be at the opposite end of the spectrum.
Tom