OK, most GM efi systems use a Bosch design as do most manufacturers. a vacuum controlled pressure regulator in these systems uses manifold vacuum to LOWER the fuel pressure at ide and at cruising speed
the lack of vacuum will cause the fuel pressure to increse to whatever the maximum designed pressure for that system is. later systems incorporated an ecm controlled vacuum solonoid in line to block the vacuum at startup under hot conditions to help eliminate "hot soak" hard starting. this too causes an increase in fuel pressure and thereby allows MORE fuel to be injected durring the injector pulse.
there different ways to increase the amount of fuel injected, the most obvious is to install larger flow injectors. but one can also increase the amount of fuel pressure and gain added fuel. another way is to make the ecm think that the engine is cold and this will cause it to go into cold start enrichment, doing this will lengthen the injector pulse which along with higher fuel pressure will accomplish the same as putting in larger injectors.
all of this does not, however explain the original question as to why he is experiancing fuel shut down under boost.
assuming that the system used was intended for a normally asperated (nonturbo) application, AND assuming that the system has a MAP sensor, the answer is simple, the MAP sensor senses the boost and the ecm interpets it as a backfire and shuts down the injectors. something that is designed into the system to prevent engine fires.
a POSSABLE remedy is to get a MAP sensor for a Saab 900 turbo and try running it to see if it will allow the ecm to interpet things correctly. the systems used on Saab are also a delco system and should be compatable.
good luck