As I've written in the ethoslab mob elevator tutorial those are the best (imho) was to improve the system. I've found a leak too; slimes won't swim up for some reasons: big slimes are larger than 2X2X2 so they can't fit inside; small slimes can't swim inside cause they're 1X1X1 size and that's a little fail for that great system;i don't know if it's possible but i think that when i'll have some time i'll try to do it; use a boat loop to transport small mobs up. What does that means? 1X1 wide tunnels with a boat inside a water column that can bring mobs up then falls inside a secondary loop that brings it under the mob and then up again so mob get "lifted" by it; i'll just have in my head the system to kill the mobs at the top; the problem is build it :) Thanks for watching, thanks to Etho for that wonderful system, thanks to Risugami and his mods, and last but not last, to Notch for that great game :D ps y slimes spawns again; in rare sectors but they'll do, i got luck and my spawner plats are in one of those areas; slimes spawns with or without light does not matter; big ones damages you, small ones simply push you hoping you've lava behind you (freaking little jellish bastards -.-) and they still drop that useless slime goo. But it's funny to hear "squish! squish!" xD