Hi
As stated in another post, the shielding gas for GMAW is very critical. Not only does it "shield", but it can have an effect on the penetration, metal transfer mode and chemical composition of the weld. (Probably forgot some other influences.)
You state that you are welding spray transfer. Under normal circumstances, you would not weld spray transfer with a shielding gas with more CO2 than about 18%. (If I remember correctly.) This means that going to the 75/25 mix will significantly affect your whole welding set-up.
It is so that in general the addition of CO2 increases the "heat" of the welding arc, but above around 18%, it inhibits spray transfer which then again starts to decrease the penetration effects.
You need to match your gas to your job. If you are welding thinner members, then a lower % CO2 is better. When welding thicker members, then a higher % CO2 is better. If you want to weld spray transfer, you must have less than 20% CO2. If you want to weld dip transfer, then the 25% CO2 is a good gas.
Hope this helps
Regards
Niekie Jooste