recon

Just made a couple platforms at y240 and left the game open about 30 blocks away from it for 30 minutes with time always set to night to ensure low light levels.
About 8 mobs spawned there. 

@Hraefin I never mentioned playing on a server 😉 

I'm currently experiencing the same issue with Slimes. I build 6 layers below y40 each 3 blocks high.
I made sure surrounding areas are lit up properly so no other mobs are able to spawn.
Waiting about 30 minutes about 30 blocks away from the layers gave me exactly 0 slimes.
I tripple checked with different sites if it's a slime chunk. I even had a single slime spawning there when building the farm.

Edit:
After checking the farm again for another hour, I had some slimes spawning. About 1 every 10 minutes which is kinda frustrating.

At least give us an option to disable it...

Still in 1.7.4 pre-release

So just for clarification: I got a 1.5.x world on my smp server which is limited by borders (no new chunks possible).
Possible that horses are spawning in this world?

Wither does spawn.
I'm pretty sure you are playing on peaceful.