I found the code to make wool blocks break faster with shears in the ItemShears class:
public float getDestroySpeed(ItemStack stack, IBlockState state) {
Block block = state.getBlock();
if (block != Blocks.WEB && state.getMaterial() != Material.LEAVES) {
return block == Blocks.WOOL ? 5.0F : super.getDestroySpeed(stack, state);
} else {
return 15.0F;
}
}
It checks specifically against wool, and the superclass method in Item just returns a constant. (As far as I'm aware, I can't override this item method of shears from my mod block class, since the shears handle the speed, not the block.)
Is there a way to get a custom block to break faster with shears, like wool?