To be fair, Deidara said
probably
So at worst from this statement, Sasori is only a little stronger, and eventhen judging by the context, he may have said that to draw Kakashi's attention away to handle Naruto alone. Considering he knew of Sasori's heart weakness and that he had a trumpcard he only wanted to use as a last ressort against Itachi, it makes that statement even more suspicious. Kisame also said after he died that Deidara was in the upper echelon in the Akatsuki
I think the mid level akatsuki members(Deidara, Kakuzu, Sasori, Kisame) are all close, with a battle between them depending on match-ups
They are below the dojutsu trio though. Overall i'd probably rank them this way then
1- Rinnegan Obito
2- Nagato/Pain
3- MS Obito
4- Itachi
5- Orochimaru
6- Kakuzu/Kisame/Deidara/Kisame(depends on match-ups)
7- Konan
8- Hidan
9- Zetsu