Consider this!
If you use only one set of welding parameters. You will be able to observe a noticable difference in the characteristics of the deposited weld bead when you change filler wire diameters from 0.8 to
1.2mm or to 1.6mm, and visa versa, simply because, of the increase or decrease in the mass of filler metal! Especially if you do'nt change the wire feed speed or the travel speed. Remember this: when more mass is introduced into the weld pool then, more energy input is required (joules/newtons), and travel speed in order to maintain weld size, penetration, and reinforcement. Adjustments in the volts/amps, wire/travel speed have to be made in order to maintain consistent weld profiles.
You also have to consider whether or not the application is performed using semi-auto, mechanized, or robotic guidance controls. For instance, if you use the same welding parameters with semi-auto, you will see changes that fluctuate according to the variations in maintaining consistency of the welding parameters due to the "slight" variations in operator position, arc length, travel speed, etc. These and other variations which can occur in the semi-auto mode can be minimized when changing to the mechanized or automatic mode of guidance control, and even more so when using a robotic mode of guidance control. Finally, different manufacturers of mechanized or robotic systems have different levels of repeatability!

In order to achieve consistency in the characteristics of the deposited weld bead, you will be forced to make certain adjustments in your welding parameters when you change from one diameter to another!!!

Also, when switching from a Metal Inert Gas to a Metal Active Gas, you will definitely see a difference when you only change the diameters of the same composition filler wire.

You will definitely see differences in weld deposit characteristics when you change from one method of metal transfer to another!!! Eg. short circuit to globular to spray when comparing MIG to MAG!!!
If you need more information, check out: www.weldreality.com .
Ed Craig runs this website. I believe it's one of the most informative websites pertaining to "synergic MIG-MAG" welding around period.
I hope I narrowed it down for you. I hope to read more from you soon.

Respectfully,

SSBN727 Run Silent... Run Deep!!!