New to the Forum and I've got a 2009 G8 with an issue it has just started recently. It was built and tuned about 4 years ago, always done very well....
I wouldn't revert the training you've done just yet. Try maxing out the thresholds 1st and see if you still show ~20% trims. If you do, then maybe it...
Definitely check the cam sensor too. May be part of the problem. This may just be my experience but Delphi cam sensors are just bad...Ive had 2 brand...
What fuel are you running? I run 93 pump gas with up to 10% ethenol, thus I set my Stoich value for 0.0709 and it seems to work well. I also have NN...