Curious, you dont think its a back pressure issue. I know the general consensus is turbo cars run better with little to no restrictions in the exhaust, but i know on Mazdaspeed cars we couldnt usually de-cat the exhaust because it would cause compressor surge. I dont quite remember if it was directly related to the decrease in backpressure or just a byproduct, but there was issues with an extremely high flowing exhaust and a "somewhat" small turbos.
Who knows, ill follow this closely, it has my interest.