Sounds like a boost control issue - Solenoid failing or Vacuum hoses disconnected or connected to the wrong ports maybe - Symptoms sound like what I experienced
(Prior to fitting the Alloy Manifold)
r.e the Alloy Manifold - I have a Positive view since fitting mine - along with a K&N panel filter it sounds Good too
Alloy is more tuneful than Plastic
My only Technical Concern would be - I'm pretty sure there is a slight port size mismatch between the VVC Head and the standard K-series Inlet Ports
I haven't noticed any compromise since fitting mine
I accept it may well be the Placebo affect
However I did feel that throttle response was better(even prior to the K&N filter)
I'm sure the VVC Plenum is bigger volume wise - maybe this affects turbo spooling ?
Paul.