if you are running the OEM BC or a 3-port then yeah.. that could have something to do with it.

 

Yes running a profec-b

 

if you were running a boost solenoid that is tied into your ECU, then yeah... the ecu would be seeing no boost and fueling accordingly while the turbo is actually forcing air in. that would make it go lean.
but an independent boost controller should not have an effect. however, a disconnected turbo signal line should make you over boost because the WG wouldn't open.. so just be careful of that.
if your lean condition was only under full-throttle at full boost then that the vacuum line could have been the issue. but you would know if you were overboosting that much.

 

question: when the car goes lean and looses power, does it fix itself when you let off the gas and get back on it? even for a little bit? i ask because of a friend of mine has had that same issue for about two years and nobody has found a fix.