Maybe this could help you getting 16-bit using 2 PWM outputs?
Resistor/PWM hybrid DAC
This is a good explanation on how to combine two PWM outputs in order to double the bit resolution. So combining two 8-bit arduino PWM outputs you can get a 16-bit PWM output! Remember Arduino's limits if you plan on going down this road. I mean, the Atmega chip is just 8-bit...
