I believe some people have had luck going through the process of calibrating the PID factors for the bed. Sounds like you've already got pretty robust wiring. Short of the PID tuning, your next option may be to use a 12V automotive relay or a solid state relay as a more efficient switch for the heater power. The MOSFETs used on RAMPS aren't exactly the best possible switches, and you lose some power in the on-resistance of the MOSFETS.