Hi there everyone, thought i'd sign up to show the solution i've been using for this.
It all starts with your basic big spawn box, with a slanted roof, like so :
On the inside, a fairly simple setup with two tier 5 enderman spawners, vines coming from the slant in the roof, using string to stop the vines going past a certain point, this is to ensure all the endermen are a one hit kill.
Moving downwards, on the floor I have a load of Aerial Faith Plates from the portal gun mod like so
They are all setup like this
And they all land next to my handy attack turtles for xp harvest
Before the latest update the jar could hold seemingly infinite numbers of levels, but since the update it stops at 30, This design could do with a small update to get a few more Jars in.
Hope this is of some use to some of you
Edit: Forgot to mention I havent bothered to collect the pearls from the turtles in this instance, because i have a tonne of them ,in this case there is lava behind the attack turtles where all the pearls are getting thrown.