Willow Springs (Big Track) will really test your skills because a mistake in 8 or 9 could have major consequences, it is also a very fun track to drive and has no noise restrictions that both Laguna Seca and Sears Point have.
WSIR is VERY fast. 9 is a very high speed sweeper which is probably scariest for those more comfortable with the track and therefore take that turn very quickly. Personally, I've almost lost it at 3 and 4, probably the slowest parts of the track but very unforgiving if throttle and steering inputs aren't spot on.
Most have probably seen this, but here's a Japanese review of the GT at WSIR
http://www.youtube.com/watch?v=0VSyO7PciLM