We use 3/32" Esab E70T-1 FCAW wire w/ 100%CO2 and get the penetration required on fillet welds. I read some of what you posted in the other thread on this subject, but it sounds like you'll need to use a drag angle rather than a push, this will do several things for you, a drag angle will help you build fillet size by using the arc to help hold back the puddle (so it will build in size for those 5/16" fillets) and you'll need to stay out on the leading edge of the puddle and not let the puddle run ahead of you or you'll leave that line of slag you were talking about.
As far as the larger wire being more harmful to your welders...Like I said we use 3/32" wire and I had a test conducted to ease our welder's concerns. I needed a baseline to see where we were as far as exposure levels, so I had several welders wear the filters (clipped up near thier collar under their shield) to gather the fumes mostly like the welder was breathing and they wore these things for an entire shift(tests were performed by an outside testing lab). By doing this we were able to gather enough info for an 8 hour weighted test and compared the data to OSHA's allowable limits. Our shop has fairly tall ceilings to accommodate overhead cranes and about 60ft wide bays that are several hundred feet long, and we were way under those limits set by OSHA. I have a hard time believing that you are overexposed in a typical structural steel shop bay by running .052" wire vs .045" wire provided the welders aren't couped up in a small room without ventilation. Exhaust fans can be installed up near the ceiling to pull polluted air away and draw in fresh air without disturbing shielding gases if needed.
John Wright