I have not heard of turning up heat meaning voltage.
Yes wire feed speed and voltage are separate. Voltage increases or decreases the arc length and therefore width. Turning the "heat up" has little if nothing to do with voltage, if you are welding you don't even set voltage till wire feed speed is set. That 2v difference you are talking about can make a big difference in weld appearance and whether it has wetted out. Amperage is what consumes the wire not voltage.
Unless you have a machine that displays amperage, you are using the hokey little voltage knob to control your "heat" input. Any hobbyist machine will not display amperage. Once you get into the industrial machines and critical welds, sure, but even at our shop we just dialed it in through voltage and a good eye and ear (old machines with the wfs notches all worn off). 30v and a good hiss and you knew you were at spray transfer, and off you went.
Voltage will determine whether you have a flat or ropey weld, low voltage is a shorter arc, and high voltage is a longer arc, blah blah. But without sufficient voltage, you're not going to obtain the sufficient amperage to consume the wire at the rate necessary.
Try to weld 1" plate at 15v and whatever speed you choose. You're not going to create sufficient fusion, because you can't achieve the appropriate amperage. Period. So it would be fair to say that there is a critical relationship between voltage and amperage. Voltage is what gets you into that usable range of wirespeed to achieve the necessary amperage.
I can try to clarify my thoughts if they seem a little wonky to follow.
I have not heard of turning up heat meaning voltage.
Yes wire feed speed and voltage are separate. Voltage increases or decreases the arc length and therefore width. Turning the "heat up" has little if nothing to do with voltage, if you are welding you don't even set voltage till wire feed speed is set. That 2v difference you are talking about can make a big difference in weld appearance and whether it has wetted out. Amperage is what consumes the wire not voltage.
I do not have a hobbyist machine so I think what is lost here is which transfer method we are talking about... you are talking about spray and I am talking about Short Circuit. So yeah there is some differences between the two transfer methods. Wertles used SC and the response was to turn up heat, i just have never heard of voltage being used to turn up "heat". So his WFS may be fine for the metal he is using although it looks a little cold, he needs better parameters cause im sure it didnt sound like it should.
Here are some of my best MIG welds.
Those welds look strange. They have a TIG'ed Aluminium look to them. Did you stop and start a bunch of times? Never seen a MIG weld with a rippled effect like that, looks like a bunch of tack welds overlapping each other.