You might get a nice even hunting pattern at 40yds with your factory full, it's worth 1 shell to find out.
TSS is not as finicky as lead, if you decide to go with a tighter constriction TSS patterns good with Carlson and Trulock chokes.
I hunt an older 20ga with a fixed xfull .585, it patterns tss 8's or 9's well beyond 40yds