I have 40% on my Z so I'm sorry I can't answer your question BUT I had 20% in all my previous cars (including 350Z) and it's pretty decent. You can still see okay at night. Backing up does get a little tricky at night but I never had any accidents. Did you check this thread:
Post Your Window Tints! - Decision guide for the non-tinted!. feel free to ask those who currently has 20% tint on their car. thanks.