Yeah it is, plus he tanked Chouji's spiked boulder without a scratch which made a bigger crater than the Iron Sand did with the same method. so he should be able to tank the hits from it no problem.
Alright interpret them how you will I guess, I personally think they can break through his defences and is what puts Kakzuu above him..bare with me while I get 'em.
Also, Sakura broke the 3rd Kazekage puppet earlier on in the fight before Sasori had to resort to himself and the 100 puppets. So it's not like the Iron Sand will remain throughout the fight like Gaara's sand, keep in mind it's simply a part of one of his many puppets unlike Gaara. It's not really too much of a matter of 'how will they penetrate the iron sand' because after the third Kazekage puppet is defeated there's not much else in terms of beating Kakuzu.
1.)
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Able to blast through hundreds of these trees effortlessly, The trees that would take like 20 people to stand around the trunk in a circle and hold each other's hands to reach all the way around the tree. Which you can see in the surrounding chapters/pages when Kakashi/Shikamaru/Hidan fight around the trees and in
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panel. Not sure how effective will be against his sand shields but shoul easily be able to blast through puppets(including 3rd Kazekage)
2.)
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Again blew the trees apart, Kakashi(not to mention team 10) was a distance away and the huge blast still forced them to block, also ripped Kakashi's clothes off completely. Not sure how effective it will be against the Iron Sand by itself but it should easily break puppets(including 3rd Kazekage)
3.)
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Solod Shikamaru and Ino with 1 hit, forced Kakashi to intercept and block it, he had to use 2 Raikiri in each hand to neutralize it. This should easily destroy puppets or at least pierce through the sand, and Sasori doesn't have the speed to dodge stuff like this(and I could also get speed feats from Kakuzu if you would like). Most puppets or masters remain statinoary, and as well as during the fight with Sakura/Chiyo they were in the open the entire time. If he fires these I don't see a realistic counter, even if he blocks with Iron Sand he has 4 other "people" on his team to attack with and overwhelm. I see this breaking his puppet(3rd Kazekage, is mainly the one that has to be destroyed ofc) without tooo much trouble and eliminate the iron sand problem, but it can ealy break puppets by itself of course.
4.)
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Just what it looks like in the pages, doesn't need much narration. Took a spiked boulder head on which smashed a gigantic crater into the ground, much more than you could say from the
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, which although impressive can't really compare.
5.)
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Easily the most impressive/powerful, huge destructive tornado of fire/wind, combination of both attacks. As you know wind fuels fire, it took a water and wind technique in combination to defend against it, doesn't really need much narration either, should easily destroy huge chunks of puppets at a time.
So anyway..to boot with all this, Sasori's poison is ineffective, and Sakura got the upper hand, fooled Sasori and broke the third Kazekage. Along with attacks like Raiton:Gian which are really too fast to be responded to easily , the third kazekage puppet should be broken one way or the other and after that it's pretty much the end of things or the end of the 'bypassing his defences". Not to mention, these are not one-use techniques and can be used over and over for mass hax/advantage. Sasori still stands a chance though, not too much of one IMO but again, it's mainly because Kakuzu is a horrible matchup for him.