Technically, the factory standard rollbar is enough (see #2 in the link below). The "broomstick test" technically only applies to cars that do not come equipped with a rollbar from the factory.
Minimum Standards
That said, I bought the Brey-Krause rollbar extension as I'm 6'2" and my helmet sits flush with the factory rollbar just to have a little peace of mind as I'm out on the track.
*Edit* Just realized that what I posted pertains to Porsche Club of America DEs only. If you're talking about actual racing (not sure what sorts of events NASA puts on), or through another organization, they may have completely different standards.