I am trying to build a mob system where the pistons push the mobs. The screen shot below shows my current design, but I'm thinking there has to be a more space/redstone efficient way to get at this. Or is this as good as it can get?
You can make your device smaller if you replace the piston with a sticky piston and put it in place of the top torch.
Put a block on it and you are done:
As ratchet freak said, you can cut the redstone use by adding a block.
Also, by alternating repeaters and single bits of redstone dust, you can cut your redstone usage by 1 per unit on average:
Finally, if you are pushing 3 block tall mobs rather than 2 block tall mobs, you can use an even more efficient system using tripwires shown here:
This increases the iron requirement per cell to 2 but cuts the redstone down to only one.