To be truthful, I don't think 5~7 grails in 4 stats is gonna help you much.
According to the JP wiki, the buff values are
Atk↑ = +1% each, +100% at Lv100
Burst Dmg↑ = +1.5% each, +150% at Lv100.
HP↑ = +1% each, +100% at Lv100
Heal↑ = +1.5% each, +150% at Lv100
No listed values for double and triple attack rates nor affliction success/resist.
Had double and triple attack at 100 yesterday but didn't recall if it was 100% or not, didn't pay attention to that detail but it was pretty dang often. Though it's most likely better to get only one of the two, since they do not affect each other and actually clash with each, unless there's a priority system with the 2.