Originally Posted by Corey
If they are the stock turbos there may not be a whole lot of top end power to be gained from increasing boost before they run out of efficiency.
True, you can see that on Alpina's dyno. After 6K, the engine starts to fall off. Interestingly, their peak torque is somewhere around 5K. 5K is where the HP peaks and stays at peak till 6K. My guess is that max boost hit 16psi at around that 5K RPM range, then drops off quickly to 6K.
Shiv did mention that he thought they could hit that level of boost in mid RPM range, but wouldn't sustain it up top because of the turbo limitation. If Alpina did indeed change the pistons, the compression ratio may be lower allowing boost levels of 16psi on pump gas.