I think we can all agree that a regular 350 will never be as valuable as an R.
This is the way I look at this, take the Demon. If I remember correctly, 3300 were made in one model year. Those things are going for a pretty penny right now even though dodge is now making the Red Eye and the Super Stock which are both within spitting distance of the Demon in power and technology.
Compare that to the 350R. If you take all the 350Rs that were made from 2015-2020 they amount to a little over 3300 cars (not sure exactly, too lazy to do the exact math).
Here is my argument, why would an R not become as valuable as a Demon or at the very least, hold it's value ?? Does the fact that the R was manufactured over a 6 year run as opposed to a 1 year run have any impact on its value?
Not an expert on how these things work and what influences the market but classic.com is a pretty good website to see market value I think. I don't think the GT350R would ever be as valuable as the Dodge Demon, the prices of the GT350s have actually gone down since more models have been pumped out and production ramped up. They were at their peak back on 2016 and 2017 but they have gone down and stabilized with the avg price at around 75k. The Dodge Demon has done way better and it has been pretty stable and rising slowly, hasn't dropped below 100k but the GT350 hit its peak at 100k and dropped down and it hasn't been able to go back up.
I also think you shouldn't compare the Demon to the GT350, as the Demon was announced to be an extreme limited special vehicle, while the GT350 was marketed as a very good, raw sports car. The GT350R is just the more extreme version.
I do think that because it was produced over 6 years takes away the limited aspect of it, and does impact its value. With cars like the classic cars like the Plymouth Cuda and the GT500 certain years are way more desirable than the others. A restored 71 Cuda is easily over 100k, but a 70 and 72 is way less I don't think that there were any substantial changes throughout the years that would make it extremely desirable (maybe the 2020 model year because of GT500 steering rack but I don't think the average joe will care about that in the future) except for the Heritage Edition. I know for the classic cars the redesign coupled with the engine options dictated the desirability, the GT350R looks the same and has the same engine throughout its lifecycle, the biggest appeal to the GT350 is the Voodoo engine which they all have.
If I had to guess maybe 2020R or a Heritage edition would fair better than a '17 or '18. Could be wrong though, if I knew this stuff I'd be rich