So.........Is the eec 4 & 5 batch fire or sequential?
FWIW - we had a 5.3 ls on the dyno once in full sequential setup. Was for my buddies efi class. He was showing the students how to test the injector inputs with a noid light. We randomly picked a cyl and told the ecu to test fire thar injector but nothing. We quickly learned in front of the class our dumbasses crossed the wires late night getting ready.
I can promise you when we fixed the issue. The motor repeated its self on the dyno with two crossed injectors just spraying fuel on the back of a closed intake valve. The motor didn't care.
I've tried both ways on my personal engine on the dyno and that alone makes no power difference including various injection timing changes throughout the rpm range.
There is however a bit of power to be had with individual cyl fuel trims which can only be had with sequential.
Most of what the oem does is all emissions and fuel economy based imho.
I just thought of something.
Your motor may be batch firing, causing one side to be richer than the other, since that is a dawback of the older batch fire systems.
Check the plug on your Cam Position Sensor, make sure the plug is intact.
Then trace the wires to that plug as far as you can, they may be pinched or internally broken inside the insulation.
Or caught up in something.
Also check any fuse that goes to "GEM"
General Engine Management.