I have a 2016 GMC Sierra with an 8 speed that has shifted poorly from first to second up shifting, and poorly down shifting at slow speeds. My vehicle has 230000 miles and I have had the fluid and filter changed at 50k which corrected for about another 30k miles, I had the converter changed at 130k with the old style fluid, however, a new part number torque converter; problem went away for about 30K miles. The fluid was changed once again at 170K with the new style of fluid and the problem went away for about 20K mile. I had the fluid changed again at about 210K miles with the new fluid and it works for about 10K miles. I now have 230, 900 miles on the vehicle and its is worse now then its ever been, shuttering at about 1350 rpm and its to a point where it bangs when shifting from first to second gear when its cold. It seems to operate the best when the transmission temperature reaches 160 degrees, however, it never really goes away.
I have a 2013 Sierra with a 6speed transmission with 333,000 miles and it shifts perfectly. As you can see, warranty is out of the question, however, I will either get a new design torque converter if I can find one, or a new vehicle that won't be GM.