I'll tell you why Team 7 takes this.
Forget about Kakashi's Perfect Susanoo for a second, it's not even needed. Obito with 1 MS has always been able to beat Hashirama; I don't care how flashy Hashirama is or how gigantic and destructive his techs are. In the face of intangibility, they're all useless, more so since Hashi has never canonically shown speed feats that would be on par with FTG (the only tech that can best 1 kamui). Now with both Kamui, Kakashi warps hashi away rather easily and quickly, making the battle:
Kakashi, Naruto, Sasuke vs Tobirama and Minato.
Now, we've already deduced that Kakshi gets rid of Hashi with relatively low difficulty, and that's before Naruto even gives Kakashi a bijuu cloak. But now Naruto has given his bijuu cloak to both Kakshi and Sasuke, ready to amplify their attacks.
Naruto and Sasuke aren't going to want to fuse their avatars (kurama and Sasuke's Susanoo, which, by the way, isn't Perfect Susanoo. Until he cannonically shows his EMS manifesting the PS we've seen from him, he doesn't have it) because they're not fighting one person that requires combined power, but rather two hokage level--possibly the two fastest shinobi ever, too.
Because we can assume Kakashi's double kamui is MS, we can assume using his techs will be straining and can soon result in blindness. Now's the time he'll use his PS (if he wanted to) because with Naruto's cloak he should be able to last for the whole fight.
If Minato's FTG barely bested one kamui, I'm led to believe Kakshi's double kamui will have superior warping speed, and his sniper will be more deadly with the cloak enhancement. (It's implied Minato cannot share a cloak to his teammates like Naruto can). I believe with constant pressure from Naruto and Sasuke's attacks, Minato and Tobirama will eventually **** up in their movements enough so that Kakashi can casually snipe them off from behind the lines.
If the hokages do end up winning, though, it's because of the experience they have over them. FTG could play a huge role, but Hashi is a nonfactor when in the eyes of Kamui.
So yeah, Team 7 takes this med-high diff