Not a bad premise, but the acting in that cut scene definitely needs some improvement. That raspy voice gets kind of annoying after a while, and some of the lines just don't sound very natural. There's also an awful lot of telling where, in a movie with a higher budget, there definitely ought to be more showing. Where Psycho Mantis starts talking about his past, for instance, I'd cut to some footage of him as a child and tell the tale in flashback if I were the director. Snake's own descriptions of himself are rather over the top, too. I'd trim those lines a bit and have him talk a little less melodramatically and a little more casually; he shouldn't really have to say "it doesn't matter" three or four different ways before the gal gets the point.
The tale Psycho Mantis tells of how he came to be what he is reminds me a lot of Doro's origins in Octavia Butler's book Wild Seed. (Near the end, it's revealed that Doro first learned of his ability to jump from body to body whenever he died, displacing other people's souls from their bodies and thereby killing them, when he accidentally killed his own parents and then his whole village.) With Psycho's story, dramatizing it would be easy. You just show him with his hands over his ears vainly trying to shut out all the noise from other people's minds, and then looking up at his father, seeing how his mother died giving birth to him through his father's eyes, and going berserk.
The language is clearly translated from the Japanese: although the subtitle's spellings are flawless and the flaws in the grammar (such as the split infinitive at -6:25) are no worse than any native speaker's, native speakers just don't talk the way these people do. "Don't regret your past--learn from it!" might show up on a bumper sticker here in America, but no one actually says things that way here. If Snake is supposed to be an American, I'd probably have him say "Are you finished moping around yet? Chalk it up to experience and let's go!" Also, instead of "More complaints?" he'd be saying "Now what?"
In short, there'd have to be some interpretation on top of all the translation, or everyone's going to sound remarkably stiff. That's what I did in those hacked cut scenes from Astyanax. That line "Come get some!" (and yes, I stole that right out of Duke Nukem) was my more robust American interpretation of the much weaker original translation: "Can you defeat me?" (From the context, I deduced that the question was supposed to be rhetorical.) That's the same kind of interpretation you'd need to do on Metal Gear Solid.