Speed is no use here. Sasori`s speed is very very fast to, and when u mentioned that Itachi has things going on, have you forgotten about Sasori`s triks and stuff his always hiding, so Itachi would never know what is gonna be Sasori`s next move.
when he died, we didn`t see the last 2 scrolls, and those must be his strongest stuff. So you are the one underestimating sasori! and "him being trained by the leader of akatsuki" Just becouse his the leader of akatsuki does not mean his the strongest, he has huge role ofcoures but being able to kill others does not make you the leader.Fighting does not solve everything. or maybe I`m wrong, we do not, it`s just to wait and see right..
But then again, we really dont know who trained Sasori, maybe it`s Madara himself :O who knows, maybe madara have ben training all of the Akatsuki`s.
This maybe a crazy idèa but it can be true to.