It's hard to say what is wrong w/not knowing his layout on the plumbing,what controls the wastegate, parts used etc. & just by talking to the guy on the phone.
"The GT3788R is perfect for 2.0L - 5.0L engines looking to make 440 - 675 HP. Twin scroll T4 turbine housing makes for quick spool up." Taken from a website.
Maybe he could join here & post some pics?
I believe he has the devided/Twin scroll T4 turbine inlet? If so, he could use the open type, not devided T4 turbine housing. That should slow it down a bit. We need to see some pics, maybe some data logs.
