My z made 388rwhp running the stock ECU, larger injectors with a modified stck turbo AFM. This was very lean and definately not safe but never the less it did make it. The car made 340rwhp for several years fairly safely with the same setup. It also obviously had other parts such as the larger turbo and intercooler to make the power but was made with an untouched 160k mile shortblock straight out of the junkyard, ported p90 head with stock cam, stock exhaust and intake manifolds. You can make decent power from the stock stuff but if you can afford a stand-alone I would definately recommend it.
JT