Owners who run high levels of boost to little or no effect on their cars may have their base engine timing set incorrectly. Although the ECU advances the timing as much as possible during operation, it has a limited range. If the timing is pulled back excessively, the ECU may not have enough adjustment to re-set it back into the correct range. Retarded timing leads directly to a loss of power.
The ability to run high boost levels and still go slow appears to be directly related to timing problems - DSMers who have run 18-20 psi with retarded timing suddenly find themselves hitting fuel cut at 14 psi, once their base timing is set properly. They also find their car is faster at 14 psi than it was at 18 psi, a direct result of timing advance. If you boost like crazy but still can't get decent times, check your timing straight away. See here for some info on how to do it.