Too much gear?
This is definitely an issue.
Op said that his et is better by .2 to .3 tenths better when he uncorks it. Sooo there is an issue!
Just to Throw Mud in the water....... My 408 has/had a 2.5" exh, full lenght, up and over the axle and dumped at the bumper. I showed NO improvement with or without the headers open.(just made more noise) Added a 10.5" collector to header(open exh) and killed a tenth.
His .2 to .3 improvement was with his old cam, don't think he has tried it with new cam.
open exh or a better exh system(3") will help his ET, But is not why he lost 4 mph.
Gears? could be. Head flow? could be Or maybe this cam just wants more timing.(ignition timing)
before and after ET slip could really shed some light on the problem. ET slip is a Cheap Dyno! LOL