You forget Iron Skin have to cast w/ physical damage every 16s.
8s belt can match 3 times with iron skin in 48s,but 6s only 2 times. it is (1200% x 12s+100% x 12s) vs (1200% x 8s + 100% x 24s) = 15600%(8s belt) vs 12000%(6s belt) thorn damage in 48s.
So it's 30% more damage with 8s belt.