The FMU raises fuel pressure dependent on boost pressure. Your FMU is most likely a 12:1, which means for every lb of boost, your fuel pressure goes up 12psi. That means that at 8lbs of boost, if we assume a 40psi base static fuel pressure, under full boost, your fuel pressure is around 136psi(8 x 12 + 40). This is a tremendous amount of pressure at the rail going thru your injectors. Even the best intank pump maxes out around 80psi, so if you try to run an FMU setup off just an intank, you will lean out under boost. As for a reason why you might want to get rid of the FMU setup, the high fuel pressures associated with them can cause injector failure, and if that happens under boost, you could potentially destroy your engine. FMU setups also tend to run excessively rich if you don't trim fuel under boost with an AFC or similar device, so you also lose power that way. If you plan on staying under 8lbs of boost, an FMU setup will probably be adequate, but if you decide to up the boost, I would not trust an FMU setup. The best option is always to use larger injectors with a standalone unit, but if you can't afford that and you don't plan on upping your boost level, then just get a new inline pump and be done with it.