Knocking around the giant Bahumut Sin,
Except this isn't a thing he did. He KO'd it and it fell, he never knocked it around. He did cut it, though.
being able to casually cut apart huge chunks of the building that was falling down on him.
That's not that great strengthwise, honestly. I maintain Spidey could replicate with an equivalently sharp sword.
he and Seph would have needed city block strength for sheer size.
Not really, they never cut anything that particularly big, and only Sephiroth sets them into motion; using his TK.
Also a lot of the cuttting that happens in this movie is amped by materia/nonphysical strength related. You see things like Sephiroth making cuts longer than his sword, and we know he can shoost magic out of the damn thing anyway. Also, the fire and such is kind of an indicator too.
They're fast and strong, but physically I'm not seeing them being better than Spidey. Seph would crush him because TK and other powers, but Cloud's a physical brawler, I'm pretty sure Spidey can take him. mmm