Why do you think it will cool down Venus? And what's the chemistry behind getting rid of the SO2?
Cool down in a couple of ways. One, ice. It melts. That takes heat. Two, gets rid of the SO by the impact blowing it right off the planet. Three, it'll also set up a water cycle on a planet that currently doesn't have a lot with as much water as we have here on Earth. Water vapor clouds will block a bunch of the IR coming in from reaching the ground. That alone will reduce surface temps.
Just off the top of my head....