That's a method I don't ever use because I never enslave. If I need time to convert a new conquest, I usually just exterminate.

It is, however, a very viable way to get population into a 'slow grower'. I wonder, though, if importing slaves has the same effect as moving peasants in and disbanding them? Most of the time, you get a huge negative growth factor and increase squalor to the point of risking plague. You have to have a base growth factor like farms and such to stay away from negative growth.