I have been trying mine up to 3.8 Volt, after that computer crash.
From what i notice is that Cas2 will gain most of the boost from higher volt.
~3.5Volt seems to give most boost at Cas2.
Cas 2.5 runs best at 3.1volt.
Cas 3 dont like above over 3.0 volt.

I think it is this way, have only noticed after messing with a booster.