If it wont rev up past 4kRPM:
1) running out of air due to turbo/intake air limitations
3)running out of fuel for any number of reasons
4)mechanical problem in valve train due to loss of valve control, usually a spring problem
I have a car in the shop with the first PP TBI system I have touched. The edit software is kinda clunky, and this car refuses to run the same for 5 minutes at a time. It also has the PP Fuel on Demand returnless pump setup.
I think that an A/F of les than about 12.0 is going to cost you horsepower, plus you cant count on the O2 to respond like a dedicated A/F gauge is capable of.