When tuning your BMW N20, N55, S55, N63TU, S63TU it is recommended to go with 1-step colder spark plugs. We've found the NGK SILZKBR8D8S (97506) in the picture attached working great when gapped to appropriate spec. If you'll be running less than 30psi of boost we suggest a 0.022-0.023" gap. We've gone to 0.018" on some cars with 30+psi but in most cases that isn't needed and will cause rough idle in some cases. In most cases though we just go with the 0.022-0.023" gap and call it a day.
One great source is from: http://www.sparkplugauto.com/
NOTE: S55 OEM plugs are highly prone to cracking even on stock vehicles. Some have used them in the past on their N55 motors without realizing the heat range is identical and they're in fact of worse quality than N55 OEM plugs. We don't suggest using the S55 OEM plugs when tuning your motors and the NGK SILZKBR8D8S (97506) have been found to be far better.
For an instructional video on how to properly gap a spark plug here's a link to a YouTube video by NGK: