Can't reduce SIM808 Power Consumption
Ok I finally could get the sleeping current advertised on the datasheet. 

First, I removed the buck converter and supplied 4V directly to the module so I get better efficiency and removed the supply current needed for the buck converter. That allowed me to strip down a few milliamps, but the true cut down in power consumption was upgrading the module firmware. After that, the power consumption in all states went down dramatically. I had to flash a firmware without Bluetooth support to get the lowest power (even disabling Bluetooth I couldn't get to the sleeping current advertised).