Well, I'm not saying the aerial slashes weren't deep.
But, like I said, there was nothing stopping him from exerting just the tiniest bit more force to completely take off Cloud's arm, instead of just cutting in deep. Also, if we embrace the whole theory that Sephiroth intended to have the foot-to-face-to-roof be his final attack - well, I don't think he really cared whether or not it went through his face. I mean, the impalement toss should have done the job, but Cloud's just so damn resilient...
When (if? can't remember...) I used the term 'playing around', I don't mean literally like "playing" - if anything it's like 'playing' a game of Russian Roullette. Totally serious, totally lethal, but there's this twisted insane 'play' aspect to it. He's really not like a cat pawing at it's prey, because he isn't just some innocent, mindless cat. He is trying to kill Cloud, he's just being a total sadist about it and enjoying every minute of pain he can get out of Cloud before he gets bored of it.
I admit that at the beginning of the fight, the battle is much more evenly matched, but I don't think Seph's really going all out. After all, he doesn't even get into his one-winged form. Once they get on the rooftop, though, Cloud gets fatigued, and if anything, this is the only part of the fight that I'd say Sephiroth decides to 'play around' at all - punching him in the face, tossing him into the air, and punting him through a wall, when he could have been stabbing and slashing.
Cloud is completely defenseless after Sephiroth has him impaled, and Seph could honestly have done anything in the world he wanted to Cloud, and Cloud wouldn't have been able to stop him. The fact that Cloud still tried to defend himself from the face stab is a testament to his tenacity, but that was a desperate grab at the blade, and, like you said, it still nearly killed him.
Had it not been for Cloud limit breaking, there's no telling Seph would have kept fucking with him or would have just finally ended it. Personally I don't think Sephiroth cares whether Cloud lives or dies - he likes seeing Cloud in pain, but if dies, well, that's good, too. It's like a win-win situation for him.
If these responses are a little late, I'm 'studying' for finals, so please pardon my latency.
Also, Cloudy, the "Sephiroth's big dick on campus" thing had me lol'in for a while. Thanks for that.