I don't think this is the direction you're going for, but if you are looking for longetivity mods, I would recommend:
- Adding the BMW Performance Power Kit (Why? Because of the upgraded cooling fan, pump, and secondary radiator)
- Replacing the existing oil cooler with a larger capacity/more efficient unit.
These things won't really improve power (much), however it will keep you out of limp mode if you are really exploiting the extra power the tune will give you.
I run stock, but some have said that the diverter valves leak when you start making larger amounts of boost. If this is true, then you may want to upgrade your diverter valves.....Again, won't look stock though.
If you subscribe to the theory that the stock n54 intake is junk, then you could use a drop-in filter for a few extra cfm, or replace it altogether (but loose your stock look)