Great build plan. The Precision 6266 is a solid choice for 600hp — it spools reasonably fast for its size and has headroom to 650hp if you ever want more. One suggestion: consider the Precision 6466 instead. It is only slightly larger but the extra exhaust housing size gives you better top-end flow without sacrificing much spool time. On the N54 with a good manifold design, the 6466 should be fully spooled by 4200rpm.
For the wastegate, the Tial 44mm is good but make sure you get the correct spring pressure. For a 600hp target on the N54, a 10psi spring with electronic boost control via MHD works best. This gives you a safe base pressure with the ability to ramp up to 22-25psi for full power.
Following this build closely. The N54 single turbo platform is one of the best power-per-dollar setups in the BMW world.
ARP head studs and Cometic gasket are a must at 600hp — good call including those. The stock head bolts are torque-to-yield and cannot be reused after removal anyway. Make sure you get the ARP assembly lube and follow their torque sequence exactly.
For the ACL race bearings, I would also recommend doing the main bearings while you are in there — not just the rod bearings. At 135k km they are probably fine but since you are building for 600hp, the extra insurance is worth the 200 EUR in parts. Use Clevite H-series if you cannot find ACL — both are excellent.
One thing missing from your list: an oil catch can. At 600hp with a single turbo, blow-by increases significantly. A quality catch can (like the Mishimoto or IAG) will keep your intake valves cleaner and prevent oil from contaminating the intercooler.
Running 650whp on my N54 single turbo setup with a Precision 6766. Your 600hp target is very conservative for the parts you are using — which is actually smart for longevity. The N54 bottom end is strong up to about 700hp with proper supporting mods.
Key advice from my build: invest in a quality wideband controller and data logging setup. I use the AEM X-Series wideband with MHD data logging. Being able to monitor AFR, boost, knock, and timing in real-time is essential when tuning a single turbo setup. One lean spike at 25psi and you can melt a piston in seconds.
Also, the Walbro 450 fuel pump is adequate for 600hp on E85 but it will be at its limit. If you plan to run E85 exclusively at full power, consider the DW400 or AEM 400 — they flow slightly more at higher pressures and give you a safety margin.
