One thing I have noticed, it may have been mentioned by now, if you are making a 'serious' mob darkroom, I cannot strongly enough suggest using Conveyor Belts rather than Water to move mobs to the drop. It requires one iron and two redstone per 16 conveyor belts, so my 18 x 18 darkroom required a total of 64, so that's 4 iron and 8 redstone. By now, you should have a cobblegen up so this shouldn't be too hard to get. Also you have several ways of getting rubber saplings, the easiest of which is to either get slime tree saplings as a reward or get slimey water to spawn them then combine slimeball with a sapling.
Conveyor Belts will move endermen, meaning you'll get more enderpearls out of your mob farm which will make it easier to advance to vacuum hoppers. They also mean you won't need a pit for them to fall into, they can be on the same level as the rest of the darkroom. Well worth the price of admission, in my opinion.
Also don't forget a bucket of water (or milk) cycled through your crafting square nets you four fresh water (or milk) which not only stack but quadruples your output for making food with. Cycle water through your crafting square sixteen times, and you have a stack of fresh water, that you can combine with a pot to make a stack of salt. Salt + Water + Flour (wheat cycled through a Mortar and Pestle) + Mixing Bowl = Dough. Triple your food output!