I still say that the bonus to settlers should be given to expansionist civs, not imperialist ones. In fact, I am working on a mod (currently on hold) where Imperialist civs get a bonus to either their occuption timer (on city capture) or their city maintainance costs. I think this is a far better representation of the Imperialist trait.
Yeah, imo, Imp can put you in a good military production position if you aim to make the most use of the settler bonus: Put the city on something that gives you at least 2 hammers in the city tile and have other good hammer tiles nearby. A popular one is to put it onto a plains hill with another forested plains hill nearby. That can effectively make settlers even at population one. Other +2 or more hammers in city tile are: Ivory on flatland, settling on marble or stone. Not sure about horses.