Let's do math.
Lb 10x in cluster mode, shooting on a 7 (-1 to hit) for 60% chance if hitting on 2d6.
Then cluster table roll, averaging a 6.
6 locations, each with a 1/36 chance of a crit, and 1/36 chance of a head hit. So 2/36= 1/18, so 5.5% [drool] chance of head or tac on each pellet. x6 (average pellet) is 6/18, or a 33% chance one hits the head or tac (with 6 hitting).
For eac tac, there is only a 40% chance of doing anything (8+). So we can modify the numbers above with its chances.
So that 2/36 = 1/36 (head) + 1/90 (tac). That gives us a 7/180 chance of crit or head. Which is 3.8% per pellet.
So 6 pellets gives us 23% that one will crit or head on a 6 pellet hit.
Now let's look at a gauss rifle. It has better range brackets, but I'll ignore that for now. Target number of 8 (we gave the lb -1), which is 40% to hit.
And head or tac is the same 3.8% as an lb. The difference is that an lb has to hit the head up to 6 times, but the gauss will kill it immediately. Even if there is no tac, it has a very good chance of punching through armour, which the lb does not.
So yes the lbs have a higher chance, but it isn't as high as to guarantee anything, and the gauss (or any other large weapon, doesn't spread damage around.
In a battle of 4 on 4, with one side having 2 gauss rifles each (or even 2 ppcs each) and the other with dual lbs, firing only cluster, I would expect the lbs to lose almost every time.
Why? Damage grouping is a huge advantage in battletech. That is why the ac20 and her are feared, but the lrm20 is only respected.
And heck, a streak srm6 has the same odds as an average lb10, and does double the damage, for less tonnage and more ammo!