I'm going to guess it's the headers making it fall on it's face. Especially if its an L33 with the higher compression.
Your going to need to bring the pipes down to 2.5" and put some cam in there.
I have seen 5.3's with a magnuson super charger fall on their faces after adding headers and then wake up after they were removed.
Headers on theses new motors are a different story than the old 5.7's and such. When you open up the flow on these motors you had better have something for it to push.

You have flown yourself out of some torque.