Take a look at the "Cursed Earth" from the Extra Utilities mod. That should make for a really efficient and compact mob spawner.
5x5 spawn room with MFR Grinder, 2 blocks high, sticky pistons above the ceiling and beneath the floor. Then you can switch the spawner off with a simple redstone signal. If that doesn't produce enough drops/Mob Essence, then you can easily stack it across multiple levels.
If you are after xp instead of drops/essence, then I would make the spawn room larger and maybe use a shifting floor design.