I just recently was made aware of this, so I decided to inject a println statement into updateNeighborsAlways() (a method that updates a block's 6 neighbors, and is called in the redstone dust update code, I'm not sure what it's official mapping is). The code I used is as follows:

package com.camellias.mlpbim.mixin;import org.spongepowered.asm.mixin.Mixin;

import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;

import net.minecraft.block.Block;
import net.minecraft.util.math.BlockPos;
import net.minecraft.world.World;@Mixin(World.class)

public abstract class TestMixin
{
	@Inject(method = "updateNeighborsAlways", at = @At("HEAD"))
	private void inject(BlockPos blockPos_1, Block block_1, CallbackInfo info)
	{
		System.out.println(block_1 + " updated pos " + blockPos_1 + " during " + ((World) ((Object) this)).getTime());
	}
}

The mixin just injects the specified method (in this case updateNeighborsAlways()) in the specified class (World) at where ever I specified (in this case at the very beginning of the method). The parameters are the parameters of updateNeighborsAlways() plus CallbackInfo, as that's required for a mixin to work properly. The code injected is whatever I put inside the inject() method, which in this case is just a println so I can see what is updating when.

I then proceeded to place down 15 pieces of redstone dust in a straight line, and power it with a lever.

When I powered the redstone, it (the redstone dust, not the lever) only updated 105 times in a single tick, as expected (6 for the blocks around it plus itself, multiplied by 15, the amount of redstone dust).

However, when unpowering it, the dust updated 448 times in a tick, which is more than 4 times then it should be. I'll attach the .txt files containing the results from both powering and unpowering the redstone below, as well as a screenshot of my setup.

EDIT: I also forgot to mention. With only 7 redstone dust, it updated 308 times, so it appears the amount of updates it does is proportional with the power of the dust.