High wire speed will cause poor pentration(cold) and roll-over, not to mention a large unattractive bead. One company we build for doesn't like the classic welded look with the ripples, so we run hot with low wire speed to make a flat, smooth bead with lots of penatration (spray arc). I always set the vaoltage to meterial thickness then tune in the wire speed according to the exact job and type of part I'm working on. Also the stickout will have an affect on your weld, less=hotter and more=cooler.

It's really about what type of welder you are, hot and fast or slower and cooler that determines setting the voltage/wire speed. Good welds can be made either way.

The thickness of the material has more to do with how much voltage and wire speed (amps) you can use effectively. For instance, if you are welding 1/4 inch plate in the flat square butt weld position, 26 volts and 180 amps with .035 E70S6 you can make a nice flat bead with 1/16 root opening. If you increase the voltage to 30 volts it will be difficult to maintain an arc for more than a few inches.

If we move up in size to 1/2 inch plate same position 32 volts and 250 amps you will do the same thing. In both cases the weld appearance will be smooth with deep penetaation.

With the same .035 hard wire you can reduce the voltage to 21 volts and the amps to 130 and weld .072 tube steel without much burn through. Tthe bead begins to build up and becomes more rounded as you lower the voltage and amps in relation to the thickness of the material you are welding. You could raise the voltage and leave the amps alone and move very quickly to achieve a flatter weld with less penetration.

The nature of the gas you are using lend itself to a wide variety of weld shapes, depths and looks. Strength is in direct relationship to the mass of weld material, penetration, and the thickness of your material. It has been my experience with the 75-25 mixture that the weld strength is always more than the material.