Can it pierce sand defense? Of course it CAN. but what makes Gaara's defense different from Kakuzu's iron hide or Raikages' v2 armor is that BECAUSE it is sand, it is constantly changing, Gaara doesnt just sit there and make scaredy faces when people get through his defense anymore like he used to against lee/Sasuke.
IMO there are many, many jutsu with the ability to break through the sand, its SAND afterall, the whole point of it is that he can change its shape and form and density however he wants.
ANYWAY, yes rasenshuriken could easily take Gaara by surprise but i dont think it would STOMP or RAPE him. his sand doubles as an impact buffer even when he DOES get hit outright, and theres always the armor of sand. Gaara is also the LAST person you want to throw a projectile weapon at