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 N54 Alpinweiss/Coral Red/Motiv HTA 3586r Tial .82AR/Motiv Port Fuel/Meisterschaft Race/Berk Resonated/ER CP/Synapse/VRSF FMIC/Rob Beck PCV Valve + Cap/ST Coilovers/M3 FCA + Tension Rods/M3 Subframe Bushings/M3 FSB/AA Strutbrace/DINAN Camber Plates/Apex ARC-8/Project Kics/VAC Hubs/Rogue Transmission Mounts/Alpina TCU Flash/Icarbon/#Tee CF Stuff/Kerscher/BMW Performance