It's just because I'm still stuck indoor for a moment and don't feel the need to go further in optimisation.
But going higher voltage is not always a good bet ... it's alll about how and where you drive, and what the servo can handle.
The Willys will be light trail truck, no need to provide 6v or more, that'll be more power provided for the servo to use to die prematurely when stuck ... better be underpowered than dead.