The peak efficiency of a turbo has nothing to do with the gear you are in. It's just the point at where the turbo will not make any more power, usually do to the air charge becoming to hot. If your turbos peak is 25lbs, then its peak is 25lbs in first, second, third or forth.
As far as having different boost in first second and third gears, that is either programmed in you chip or you are having a problem somewhere else.