You could always go with a quicksand system.
Jazz Hands by
Saice, on Flickr
How would you collects the drops, underground network of transposers + tubes? Also, do transposers/filters need to be pulsed to pick stuff up?
Actually, I think I've been over-designing this [with a somewhat vanilla mindset] to try and maximize spawn potential. Ex: my XP Blaze grinder starts with a 10x10 area around the spawner and piston-cones [Etho's Ice Cream Cone design] down to a 1x1 kill zone using 2 turtles. It's so effective, I can only leave it on for 1-2 minutes before I start to seriously lag [2 turtles cannot keep up]. And this is the only mob I required T5 for something other than the redstoned-cage ability.
Finally found a video with multiple redstoned cages and the spawning area is a normal 8x8 with no drop-zone for the mobs [the bottom of the spawn zone is right above the water flows]. Water pushes the mobs [in this case Blaze] 8 blocks into a line of turtles at head-height with transposers underneath to collect the drops. The brief time his system was active, it appeared the turtles had no problems keeping up with the mob stream. I know the water was helping, since they were Blaze, but this design was also much more compact than what I've been building.
I may have to use quicksand for the passives, as I'd like to keep them together [cows and chickens] unless I put the turtles at floor height with the collection system immediately the turtles.
Need to iron this out before I setup the Enderman's spawn area, as that one gets covered in water and redesigning afterwords is a pain beyond measure.