[Freon]

I would not bother with the throttle body spacer. It is junk.

Not sure you can actually get a vacuum signal on an N55. It's a Valvetronic engine, so there is generally little vacuum created by the throttle plate. I think it's just there as a backup in case of Valvetronic failure or for special circumstances, maybe someone can elaborate on specifics. There's a separate electric vacuum pump to supply vacuum for things like the brake booster and boost control.

Might read these posts or try searching more for "N55 boost gauge" on Google.
http://www.1addicts.com/forums/showthread.php?t=438101
http://www.e90post.com/forums/showthread.php?t=513542