Core XY is weird, some moves both steppers move for double torque, some moves one has to spin twice as fast as normal as one may be completely stopped.
Holding torque is different than torque at speed, as at speed, the stepper loses a ton of torque.
If current is too low you can overshoot and develop ringing, loss of steps and inaccuracies at corners. And yes, it increases the accuracy during acceleration and deceleration.
I would bump it up to at least 70% the 70% figure CAN be gone over if you have cooling/ if it isn't overloaded.
I have my E stepper close to max amps, and it runs luke warm, but I also have a alloy print head to wick away heat.