I found a skeleton mob spawner today, so I decided to make a traditional trap used on mob spawners. For those who arn't familiar, the traps look like this:
Basically a bed of water sits under the spawner block, and it brings the mobs over to the glass wall where they drown. I decided to do a 15 minute test run of the farm. I got 50 bones, and 54 arrows. Meaning that the spawner averages about 400 items an hour.
This is a lot better than I originally thought. Some of the larger mobfarms I have built have only had item rates of 800 items an hour. But these took 6-10 hours to build instead of 20 minutes. The one downside to these kill rooms is that they only work with zombies and skeletons, meaning you can only get arrows, bones and feathers from them at this rate. This is a major setback for me because I like having plenty of TNT for defensive reasons.
Overall, this design is good if you want to get a lot of arrows fast or need bones for growing crops. If you need lots of gunpowder for TNT or string for dispensers, I would stick with larger farms. I'm working on a new design for my farms, i'll share it later in the week, when I finish it.