I disagree. I dont think you will see a large discrepancy in price simply because one can buy the same thing and there are lots of them. Now if the 15 was the only one getting the 5.2 flat plane crank it would be special different and its value would sky rocket. That is not the case, the 137 does not make it special since there will be 10k plus units with the same options. 2009 GXP, 2003 RS6, 2011 GT2RS are all great cars because there are less than 1k made(sold in america) and they were only available for one year. Not the case for the GT350.
Can you name one MY vehicle in which its price well is above other years with the same body and drivetrain? Are the 2k zl1 built in 12 more valuable than the 8k zl1 built in 13?
The 1964.5 mustang and the 1963 split window Stingray corvettes.
Neither of those were "special edition" trims either, but they both sell on average for more than the other years with the same model.