That is true, cordite. Oft times, heavier charges are less accurate due to rising pressures causing blowby and patch burning. A larger ball and same patch might just be the 'ticket'.
My bro's new Lang rifle, a 16 bore, will shoot well with 80 to 100gr., and has a 48" twist.
What shoots to the sights in a SxS is what is important, of course.