Rasen Shuriken already gets a massive boost from Sage Mode or Bijuu Mode alone. Naruto can use both modes at once, thus he gets both boosts added onto his FRS, and he can use the Chou Oodama variant. Given how much damage a Sage Mode FRS did to Kurama, enhanced by Bijuu Mode as well, along with the fact its
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. It should be on par, if not stronger than a regular Bijuu Dama. Hashirama can't catch these, so it'd do some good damage to SS, especially considering the fact it'd cut and pierce unlike a Bijuu Dama, thus its more focused and thus it'd hurt the statue more than a regular Bijuu Dama would. There's also the fact that Sasuke can add Enton to it, meaning it'd explode with flame stronger than the one that was burning the Juubi.
There's also the fact that their Kurama Susanoo union is superior to Madara's since BSM Naruto>Full Kurama (When used by Madara at least) and Sasuke's Kurama cloak Senjutsu enhanced Perfect Susanoo>Madara's Perfect Susanoo. So SS isn't going to beat it when it used up its arms dealing with something weaker. They have better offense, and better defense.
It boiled down to stamina at the Valley of the End, and Naruto has much more stamina and chakra than EMS Madara, and if Sasuke runs low, Naruto can always pass him some more chakra, so after SS is destroyed, if Hashirama is exhausted, they'd obliterate him from there.
So EMS and BSM take him mid diff. Either one solos no diff post Rikudo power up.