I think that everyone's made some really good suggestions and I agree with all but I'm wondering why no one has mentioned MCAW (I believe in the US it falls under GMAW-C)? Please correct me if I'm wrong. MCAW can really only be done in flat and horizontal positions and most won't work so well out of position. If they do there deposition rates are not effective. It also may not create as much of "learning curve" as switching to FCAW and it would give a good balance between the GMAW and FCAW. Also an 0.045" MCAW will provide a slightly better deposition rate than 0.045" FCAW and better than an 0.035" GMAW.
Personnally, I'm not a big fan of 250 amp machines for FCAW or even GMAW (spray). IMHO you can't maximize your FCAW productivity on any large fabrications with a 250 amp machine (if most welds are in the flat and horizontal positions). Voltage seems to get maxed out at 28 to 29 volts. With the 250 amp machine with an 0.035 GMAW, with a 85/25 Ar/CO2 shielding gas at 220 to 240 amps, 28± volts will provide good deposition rates too. The bad news is that at those parameters welder technique problems may be highlighted. Lawrence makes a great point about proper process selection and parameters. Take a look at the deposition rate tables for the same size wire. Spray transfer with a small wire may deposit more metal than a slightly larger wire in short circuit...
Has operating factor, joint design (ie overdesign) and welding position been looked at to improve productivity? All of this will impact deposition rates and productivity too.
As for shielding gases. Here's a decent article about them
http://www.thefabricator.com/Consumables/Consumables_Article.cfm?ID=1024What you will notice in your welds is that as the argon content increases you will get deeper, finger like penetration (argon has a narrower arc characteristic). If you increase the CO2 content you will get wider penetration. Also you will note that as CO2 content increases so does spatter and fumes (there is more CO2 to break down and react with slag or glass formers as applicable). With higher concentrations of argon you can also decrease your voltage slightly for the same amperage because of its higher ionization and lower thermal conductivity.