If you're boiling the coffee (which is what the moka pot does), you're burning the coffee. I love moka pot brews, but as least be honest: you're burning it!
In the three-chamber design, there's a tight seal between the lower chamber (filled with water), and the cup (filled with grounds), connected by the siphon pipe.
As the water is heated, even before it reaches boiling point, it's being forced up through the grounds. The last dregs of water are steam-hot, but earlier is not. And the liquid water keeps the temperature of the entire pot (including coffee) at or below the boiling point.
The pressure and heat should result in a strong, rich, but not bitter, brew.
Leaving the pot on the stove after it begins to "spit" (as steam sputters up the stempipe) will burn the coffee (and seals and pot). So Don't Do That.
You should try it.
With water at 4C, puts the most water inside the pressure chamber, leading to the greatest pressure change for a given delta(T). Instead, if the water is just below boiling, the water going through the grinds must be higher than 100 C (since the pressure raises the boiling point).
It'll brew faster, and there is an argument to be made that quicker brews lead to less burning. But the T would be highest.
Boiling less quickly (i.e. using lower heat) might put the coffee in contact with boiling water for longer. So it would seem high heat and cold water would be the way to go if you want to minimize the temperature-time product of the beans-to-water contact.