True we know almost nothing of Kushina except for massive ammounts of chakra and high level sealing skills.
Without Kushina Asuma vs. Kakuzu
I'd say they're around the same level the only reason Hidan took him down was because of his immortality and the mystery of his techniques, Asuma could just use his Wind chakra and enhance his kunai and attack with huge wind blades whilst throwing powered up kunai that will pierce kakuzu, Kakuzu can go ranged mode and try to keep him at a distance
How I see it is if they get too close the win kunai will be too dangerous and damaging for Kakuzu to withstand even with multiple hearts, though if Kakuzu can keep the fight at a distance for a while Asuma will go down.
Remember Kakuzu took down Ino/Shika/Cho with help from Kakashi all at once so his basic skills are not to be looked down on but in raw power Asuma is above Kakashi by good bit.