How did it shoot before you had it threaded? Mine is the factory full. With reloads, #9 shot, it puts ~150 in the 10" at 40 yards.
Have you tried 9.5 shot? My 301T would put about 110 of the #9 shot in the 10" but I switched it to #9.5 shot and I get 150 in the 10" at 40. I tried the #9.5 shot out of the 870 and I was still hitting 150.
Can't say I did a deep cleaning. Just ran the brush down them several times and then ran a bore snake between shots.