https://blog.caranddriver.com/tested...-fiber-wheels/
~24% reduction in wheel/tire weight + a ~4% gearing reduction leads to ~8% in-gear performance difference that decreases as speed increases.
As for coast down from 60mph, a ~3% difference. Considering that the gearing change means the high-revving Mustang is more in its power band and gets a gearing advantage, I'd say the coast down is a better measure of wheel weight's impact.
The GT350R weighed 3710lbs. The GT350 wheels weight a total of 58 lbs more. That's ~1.6%. Keep in mind that I rounded all my other percentages. Anyway, if you're getting ~3% difference in inertia after dropping ~1.6 per cent in curb weight, that suggests about a 2:1 ratio weight loss improvement from wheel weight versus other weight. Then again, the weight of those GT350R tires is ~4% inwards versus the GT350 tires. That also reduces the inertia. Maybe 2:1 is generous?
So ~$16,000 to reduce tire/wheel weight by ~15lbs/corner (about 24%) yields a significant difference but nothing to blow your mind.
What I suggest you all do is get some 215/35R17 Yokohama S-Drive tires on stock rims. They'll make the car feel much faster. You get a free lowering, too.
Also see this:
http://hpwizard.com/rotational-inertia.html
He suggests 2:1 at most. That jives with what I'm seeing with C/D's article.