At that point Gaara was yet to overpower the 4th in a battle of weight. Considering that and the fact that IS should technically by heavier than regular sand as well, i'd say Sasori has a good shot at winning this.
He doesn't even have to overpower Gaara's sand, he can just mix it w/ IS to make it slower and then attack Gaara from
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, but not at the same time (w/ a short interval), in order to make the sand focus on one side and not be able to react to the other in time.
IS biggest structures shouldn't be underestimated either. They don't seem very impressive because all they did was destroying rock, but i wouldn't be surprised if they were able to bring down some strong defences. Similar to what happened
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.
Meh, but maybe Gaara has this even w/ early part 2 feats (i'm not exactly a Gaara expert), but he got much better by the time of the war, that's one of the reasons why i'm questioning myself. The other being portrayal.
Who'd say Sasori can take down a Jinchuuriki? Yet he was on a team formed especially for that effect.