Boardman feeders work if you use two per hive and reduce the entrance with them, either one on each side of the entrance or both on the same side. Makes about 3 equal sized sections of entrance, two blocked by the two Boardman feeders. Smaller entrance on an 8 frame than a 10 frame hive.
I think this idea is similar to what I came up with to reduce the entrance size. When I need to, I use a 3/4 by 3/4 by 4 right up against the boardman slid into the entrance so the bees have to enter the other side of the entrance away from the feeder. If that still caused problems a simple screen looped over the entrance so the bees had to enter the sides fixed it. the robbers couldn't dive bomb straight into the opening anymore.
I Fed too long initially, and could never figure out WHY the smaller hives didn't gain strength.. I found out (By asking questions here) I was plugging up the brood nest with sugar syrup.. they were sucking it down and storing it in every available space as fast as they could.. no place to put natural stores, and no place for the queen to lay.
I am more noob than experienced, but found that pulling the feeders after a couple of weeks to let the bees get on with it on their own made a vast difference in how fast they build up. You can always pop the feeder back in after a week, or three weeks later if they are still having trouble with numbers and gathering enough resources.