So 1 grail into +burst is equal to +1.5% +burst. 100 grails = +150% +burst

The issue here is, what does +burst actually mean. +150% +burst does not mean 'increase your burst's damage by 150%'. It means 'add 1.50 to the burst multiplier'
A 3* SR kamihime has a burst multiplier of 3.5. Adding 1.5 to that would be a ~42% increase to damage, not the 150% increase you'd assume if you don't know how burst damage is calculated.
A 3* SSR kamihime has a multiplier of 5.0 (and for an awakened SSR, it'd be 5.5). Going from 5.0 to 6.5 would be a 30% increase, while going from 5.5 to 7 would be a ~27% increase.

Moreover, our version is still old enough that the cap on +burst buffs is 300%. Shingen's provisional forest maxes out at +200%. So for those using Shingen, there ought to only be 100% left before you hit the cap. Any amount of +burst past ~67 grails will be useless for Shingen builds.