it's a bigger increase than you'd think. presuming a boss fight lasts 5 minutes you can get in around 10 CoA's, with amp curse you have 5 additional seconds to cast spells, so 2 more SBs.. also another way to look at it...
if your CoA does 6,000 dmg, with 1.5 gcd it's 4000 dpsc. with 1sec gcd it's 6000 dpsc. pretty large increase.