I could see this working, I don't really play Kassadin (or Mid in general), but having to spend 800 mana and going into melee range for a one-second silence is not OP in any way at all imo depending on how many targets you actually hit, because in higher levels of play people are pretty spread out. In fact, I would propose possibly increasing the silence to 1.25 and 1.5 by ranks 2 and 3, just to maybe make the trade-off a bit more even. Kassadin mains, or Mid mains in general, what do you think?
I think silences re-implemented on certain assassins in this way or another could be a lot healthier than compared to a low risk/high reward ability like Talon's E, Leblanc's Q, and Kassadin's Q, which were all ranged point-and-click abilities that created the "Silences are unhealthy for Assassins" trend in the first place anyway.