The only way I can see Bang winning this is if we assume Akuma will go easy on his foe, his "likely higher" rating isn't taken as concrete fact, won't use the Shun Goku Satsu, and spends the entirety of the fight engaging in physical combat, thus getting his physical attacks overwhelmed by Water Stream Rock Smashing Fist, and Encampment, allowing time for Bang to slip in Fang Interpolation or Instantaneous Effect for a quick finish. However, if he does decide to use the SGS during physical combat, his intangibility which leads into the SGS trumps every physical tactic Bang's got, including being able to phase past Instantaneous Effect.
But practically speaking, and assuming Akuma uses all his tools, he's pretty much got a huge edge over Bang. Long range, Akuma can pretty much spam his various hadouken variations, and even though they'll likely be deflected, Akuma'll prolly be able to see em coming, dodge, continue to play the long range game and possibly overwhelm Bang's defenses with continuous usage of Messatsu Gou Hadou and possibly Tenma Gou Zankouu. Or he can use the SGS while Bang is busy defending against projectiles.
I give Akuma the victory based on his long range projectile game, and the Shun Goku Satsu (assuming he does indeed use it) trumping anything Bang can dish out.