Thanks, I am having an issue with my local BMW stealer....
As Gordon (BmwCoolK1200) told your there is NO fault codes directly related to system monitory fuel pressure.
HOWEVER, there are at least 2 types of symptoms, one of them being an indirect fault code:
(1) engine behavior at various RPM or conditions (does it do it cold or warm - is it at full-power or at low-power...). This type of troubleshooting is common to most fuel-injection systems having similar design (pump in-tank with returned hose). Many later BMW motorcycles after 2005 have a non-return hose system (from fuel-rail back to tank) and these can indirectly monitor fuel pressure (like many modern cars)
(2) when you run such engine having low-fuel delivery (cause by lack of pressure / capacity), the EFI system will eventually trow other codes. BOSH Motronic we have is not OBD2 compliant and as such the EFI codes are pretty limited. But in most cases you would get a code that says the system is running at extreme compensation - trying to adjust the "too lean" mixture. One code seen would be: 4400 - Lambda-Control, Upper control limit reached
Sadly this state of affair you have seen is not only for BMW motorcycles, BUT also in many car dealers and repair shop. Many can change muffler, brakes, oil, tires, but my rough guess would be less than 50% are good at troubleshooting EFI systems.
P.S. 1: The TVA (idle-actuator) has no effect on fuel-pressure OR power at higher RPM. It will of course cause problem to maintain proper idle (cold or warm) and will cause issue to start engine cold. In fact, I have done test where the TVA was removed completely. Assuming the engine was warmed up before, you could ride all day long but the idle will be too low and engine would stall at idle throttle position (unless you manually compensate).
P.S. 2: monitoring fuel pressure (with bike running on center-stand) is not "rocket science". Even you could buy a medium grade fuel-pressure-tester kit for less than $100 (various test to be done cold and warm - some engine running - other static). And same fuel pressure tester kit could be also used on many cars and motorcycles brand. I fail to see why many dealer do not always do these tests... with such symptoms that you describe.
P.S. 3: installing non-submersible fuel hoses inside fuel-tank ..OR.. fuel hoses not rated for proper pressure is borderline criminal in my opinion.