Need help - Vacuum loss and can't resolve

[68GMCCustom]

When my truck was running this new genIV LSx motor with a carb'd setup...it ran with 15-17" of vacuum. Truck ran well but a bit rich. I also installed brake booster along with a vacuum storage tank along with other brake upgrades during the build. Brakes were awesome...could throw a passenger threw the windshield.

Then came the change to efi. Removed the carb'd setup and on went plastic efi manifold and ported throttle body, running a custom harness w/e38 ecm (t42? tcm) combo, w/4L80e trans, running a custom tune by well known tuner. That's when things went south. Vacuum has been in the 10-12" range since then and through 4-5 different tunes. Idle seems similar to when carb'd...just not making the same vacuum any more. This motor is not old, and is/was a "new" 0-hr. motor. Less then 1000mi now.

Items checked off -
Changed pcv setup - no change
Closed off any/all ports including one for brakes to check vacuum level - no change
Obtained new OEM efi intake manifold and gasket set - replaced w/o any change
Throttle body - swapped from my TBSS - no change
Also -
Pulled the injectors and setup a test bed. All injectors fired and sprayed similar strong streams
Pulled each injector connector one by one while running - each cylinder appeared to pull engine down when injector disconnected and picked back up when reconnected

I need to resolve this. While the motor is running and making good power it obviously isn't right at 10-12". I have HPTuners and have tried some things there w/o any improvement. With the vacuum at this level as you might expect my brakes absolutely suck now...not a problem a carb'd to efi swap should create!

Ideas?