Tuning is definitely a big part. When I went from the stock tune to Heaths with my GM 8 6 years ago, there wasn't much improvement over the stock one. I actually sent it back to them the first time thinking the programming didn't take because I really didn't notice any difference in power...
I got a low boost code with the stock PCM but not with the heath one. I'm thinking you should have tried the KOJO tune before you sent it back. I had a lack of power in OD with the Heath PCM and ATT. I am trying the KOJO one now and it solved my OD problem. So now I have to order my own KOJO...