If everything you stated is correct that you have checked what else could it be but the porting. Look at the plugs and see if you can point to certain cylinders or is it all of them. There are times a genius will destroy a set of heads not having a clue of what they are doing.
We had a guy come here and we Sliced a set of Viper heads for a porter so he would know how far he could go. The heads he ported are here right now to get welded and repaired he ported through pin holes. There are times you can't fix stupid. Making a port larger is not as important as changing the characteristic of flow.
Bigger is not always better. My wife told me its not how long it is , Its how long it lasts. LOL
View attachment 1716072439
View attachment 1716072440
View attachment 1716072441
View attachment 1716072443
View attachment 1716072444
View attachment 1716072445
View attachment 1716072446