No, it helps in Cloudy and Sunny Conditions.
Here's an example with a Voltaic 2W panel for my specific test stand, which is a 100W LED light.
[disregard the "cold" curve, that's before the LED test light was at full output]
For the 2W panel shown above, the 5.08V PMIC setting will produce 10% more than 4.84V would......under my test stand. I would have guessed the difference would have been higher.
Here's a graph showing how the Max Power Curve depresses down and shifts slightly to the left as the Brightness decreases (Clouds):
The voltage at the Max Power for each curve doesn't change much and is obviously panel specific.
Your 6-Volt 3.5W Voltaic Systems Panel will always produce the most Power when the load can be adjusted to maintain the Panel at 6.5 Volts, but 5.08 is our highest setpoint.
I dug through several PR's and the Source Code, and combined that with posts from @avtolstoy.
I was one of the vocal one's requesting the Disable function for System Power Manager (which is now available with rc1).
But I'm now a fan of the New API and SystemPowerConfiguration in rc1 
.