I have no idea what's wrong with your meth kit, without more detail it's tough to trouble shoot. Talk to Shiv he'll be the best resource for this. There are some folks with N54s running 20psi or so midrange, some also run aggressive tapers that hold boost pretty high towards redline. Anything over 20psi and I'd have to think you're leaving the efficiency range of our stock units. 17.3psi seems a bit low for FBO+meth, i'm currently topping out at 17psi or so midrange without catless downpipes or meth.
E88 FBO N54 Alpinweiss/Coral Red/Meisterschaft Race/Berk Resonated/Macht Schnell 3"/ER CP/Synapse/VRSF FMIC/FFTEC LPFP/HFS-4 DO Stainless Lines/BMS DCI/BMS Thermostat Delete/Rob Beck PCV Valve + Cap/BMS OCC/Cobb E60/ST Coilovers/M3 Front Control Arms + Tension Rods/M3 Subframe Bushings/Apex ARC-8/Project Kics/Macht Schnell Studs/Rogue Transmission Mounts/Alpina TCU Flash/Icarbon/Kerscher/BMW Performance/Cyba