After actuallyreading your post it appears to me thatyou aretrying to do this in the most economical way...I also take it the Mossberg doesn't take screw in choke tubes...That leaves just the Savage single shot with a full choke to try out...I would suggest picking up a box of the new Winchester 20 gauge Xtended Range 3"#5 shotturkey loads and trying them through the Savage since it has the tighter choke...I believe the modified choke is going to be to open andwon't provide a very good pattern past 20 yards...The Savage should pattern pretty well out to 30-35 yards...And yes the new heavier then lead loads will give you a tighter pattern then lead...and to answer your last question...No do not go to 7 1/2 sized shot...they may contain more shot but at 30 yards they will lack the energy to kill a turkey.

20 gauge shotguns are the up and coming gun of choice for turkey hunters...For years they were concidered to light for turkeys but in the past couple of years manufacturers have been really improving the 20 gauges performance to darnednear 12 gauge effectiveness.