I use .003 and .006. Works well. They played with the specs to get the valves as quiet as they could to meet EPA sound levels. Sound levels on a bike are much more than just the exhaust. Air intake, valves, sound that comes straight out of the cylinders and cases. All this is part of the EPA regs. Water cooling is used as much for sound damping as cooling the engine. The water jacket absorbs the noises from the valves and cylinders.
You can tighten up the adjustments a bit more but you may experience a loss of power from the valves leaking and a leaky exhaust valve can burn. I like a bit of noise. This tells me the valves are doing thier job.
Leo