He seems rather weak, and I have read comeppling arguments from The Dark Artist suggesting he is not Akatsuki level. So, is he at least Kage level?
This is a very interesting question. Now let us think of what we have to compare Sasori with, what are some of his feats:
1) His main feat was killing the 3rd Kazekage, who possessed magnet release.
2) Being defeated by Chiyo and Sakura
3) Puppet Army
4) Creations
Now lets address these:
1) The first one should be given alot more hype than it actually gets, especially when Ginkaku and Kinkaku, who we all recognize as strong ninja, failed trying to kill the 2nd Raikage in a 2v1. So the fact that Sasori was able to defeat the 3rd Kazekage in a 1v1 shows that he is at the very least at Kage level.
2) Now lets look at the 2nd feat a little more because this was actually him being defeated. And it is because he was defeated by these 2 that people question his skill because they see Sakura as being weak and Chiyo as being weak. But lets consider how strong these ninja really are.
Sakura - Direct Training under Tsunade, Extremely skilled at Medical jutsu, Monster Strong, and seems to be close in Sasuke and Naruto as far as development goes. So with that we can compare her to Naruto and Sasuke's strength at the time. Now this isnt excessively strong but it is strong enough to make a difference, and lets not forget the ever powerful plot no jutsu.
Chiyo - This is an embarrasingly underrated ninja. I dont know why people do it. First off she is a part of the Elder Council under the Kazekage. (Want to know one of the people in the Elder council under the Hokage? Danzo... Remember how strong Danzo was? Lets not underestimate people who acquire this position.) She was renown in battle. Superb medical ninja, fought with Tsunade several times enough to dislike her for constantly creating cures for her poisons. At the least I think we can liken her skill to that of Danzo's. If that is the case we must ask who Danzo and Sakura, when working together, couldnt beat? This is made worst by the fact that Chiyo and Sakura synergize extremely well, as Sakura pretty much becomes a puppet for Chiyo to use that happens to have super strenght and can heal itself.
3) Thirdly we have his ability to control a puppet army. It was said that a puppeteers skill revolves around how many puppets he can control at one time. Chiyo could only control 6 or 7, Sasori was able to control 1000.
4) Kankuro's current puppets Black Ant, Black Crow and Black Salamander were all creations of Sasori, who was regarded as one of the greatest puppet makers. So much so that even after his death Kankuro took Sasori's body as his new puppet, named Scorpion.
Now I am not going to pretend that Sasori is in Naruto's league or in Sasukes, or Hashirama's or anything like that. But to say that he was too weak to be a part of the Akatsuki is borderline blasphemous. Do you think Sasori couldnt take out Hidan? You think he would have problems with Deidara? How does a person without eyes fall for Itachi's Genjutsu? You think Kakuza could overpower 1000 puppets?
I think Sasori is alot stronger than people want to believe. But regardless that chapter of Naruto is over now and we wont be seeing him again. So lets not speak ill of the dead.