I'd like to point out that this entire line of speculation is based on the line after Cloud's flashback:
Now this is one of Cloud's memories that we can trust. It's unfortunate for the party that it corroborates Cloud's story, but...Tifa
"Cloud...how bad was I when Sephiroth cut me?"
Cloud
"I thought you were a goner...I was really sad."
Well...
If you're talking about a normal katana, it wouldn't really be a blunt strike, but merely a slashing motion that would open up the artery next to her collarbone, essentially deform one of her breasts (take that, hentai artists), and spill her entrails on the ground.
But if it's the Masamune? I kinda get the feeling she would have been sliced clean in half...this is possible with a katana of sufficient caliber in real life, so if it's the Masamune? Yeah...Zangan's "Cure Spells" woulda been too little, too late.
Perhaps he merely cut the artery near her collar bone? That would be sufficient to incapacitate a teenage girl, and would cause sufficient blood loss to be a life-threatening injury. But she hunches forward, gripping her shoulder, and then a twinge of shock runs through her back, and she throws her head back and loses her footing. Seems legit...
I spend way too much time thinking about these stories.