A Global Time Dilation setting greater than 1 does not affect destruction physics, physics always appear as if global time dilation would be at 1. Global Time Dilation below 1 does work, at 0.2 the destruction pieces are actually falling with 20% speed.
I know that to be same precise at a global time dilation setting of 2 the physics would need 2 times the computing power as on 1, but this should not really be a problem.
I am using 4.7.5.