Since you're already assuming you have a good game, and it's popular enough to make it through the Greenlight process, you don't have much to worry about. Once you have enough critical mass to get Greenlight, people will have voted to get your game there, it's highly likely that at least some portion of them will also buy the game once it's on Steam. Additionally, once it's available on Steam, you have a much larger market that will now see and have the ability to easily purchase your game.
So, yes, you can expect more marketing and thus sales. If you have Steam selling your game, you'll have more sales and marketing. Certainly a lot more than if you never had your game on Steam. I can't imagine a situation where you'd have fewer people knowing about your game or fewer sales as a result of getting Greenlight. You can find some statistics for Greenlight games here. And an article here.
Steam is typically very tight with marketing numbers. Even within the Greenlight program you don't have exact numbers for how many people need to vote in order to make your game Greenlight. It's highly unlikely you'll be able to find statistics that show before/after for Greenlight games. Steam doesn't like people sharing their revenue information about games sold on Steam either, so it's even more unlikely. There are some hints here and there. All in all, things point to the obvious, getting your game on Steam is better than not being on Steam.
With the assumptions you make, you are nearly guaranteed some form of success.