I think you are missing something important here. from chapter 672 and previous one, it appears that Madara can even regenerate his broken Ribs. Basically, Madara is currently almost immortal and unbeatable. And in case you don't understand, if we consider Naruto as only Ashura reincarnate and Sasuke as only Indra reincarnate, the result of the fight will be this:
Madara will kick them medium difficulty!
Well, believe it or not, but it is fact. Madara has transcended to RikudoShip + Juubi Jinchuriki. This make him possess the original powers that ere transmitted to Indra and Ashura. Besides, the followings hold:
(1) If we consider Sasuke as Indra reincarnate, Madara already knows all his power with PS being the greastest one
(2) If we consider Naruto as Ashura reincarnate, Madara also know his power because it should be at the level of Hashirama's power.
Thus, the only reason Naruto and Sasuke have a little chance to beat Madara are:
(a) Something new Hagoromo gave to them
(b) Naruto is Uzumaki and may have Uzumaki power as well
(c) Sasuke regain Juugo's powers and others through whatever Kabuto gave him
See, at the end, Both Naruto and Sasuke will fail with their own power-up.
This is why Naruto was given the Bijus. Only the power of the Bijus will be the true power-up against Madara because it will take Naruto to the same level as Madara.