The 9mm^3 flow rate is the average expected to feed the whole common rail. You also need to measure the actual to get the difference, which then gets added or subtracted to each cylinder balance rate. So unless you can measure the main pump flow rate do not worry about this number.
150k for LLY and newer engines is nothing. Most go 200k+ before replacing any injectors, but it's not impossible you need to replace one or two. Also, you don't measure these rates just once. Every time you run the vehicle the rates will change. Only when the rates consistently stay outside the spec range or are wildly out does it indicate a problem.
Replacing one injector at a time is typical on LLY and newer engines. One injector should not cause other cylinders to compensate, each injector acts on its respective cylinder. If replacing one injector does cause other rates to change, it's because the main flow rate changed along with the new injector, and the other injector readings are simply adjusted to the new flow rate since they are average offsets from the main flow rate.
2005 LLY 3500 CCLB 215,000 miles