I am in the same camp as those suggesting that you work with the shot shell rather than change the choke tube. I use a .654 and went through several shot shells before settling in on my choice.
In my book, at the 40-50 yard range, regardless of shot shell, you are at the edge of consistent lethal range. I, like probably many other avid turkey hunters, have killed gobblers that far out or farther. I killed a nice gobbler several years back that turned out to be about 60 yards out .... frankly I had miss judged the distance and was thingking more like 45. But I am not all that comfrotable with shots beyond 40-45 yards and try by best to wait until the gobbler closes to within 30 or so.