I live in Belcher's and it is very easy plus quick to get to Central. As penguinsix said, the flyover takes the least time (4 minutes exactly from Belcher's to IFC 2 in a taxi, where my husband works). Some mini buses take that route also. Mini buses in general are faster, but be careful to sit down quickly (their drivers drive like they are drunk). A citybus is slower. But definitely avoid the tram, if you need to get to work.
I would say it also depends upon where in Kennedy Town you are planning to stay. Find out in advance the nearest bus stop and its travel time. You can use
www.centamap.com or
www.ypmap.com