I would go for the small turbo and here's why-
AMG, now owned by Mercedes has a beast of a 2.0 liter turbo that would take a lighter weight Z into new territory. Power to weight ratio will plummet along with lap times.
SEE ENGINE ARTICLE
Nissan will be building Mercedes Engines in TN, USA by the time the Z goes into production.
SEE NEWS ARTICLE
You want to sell more cars to Americans then manage your supply chain costs based on proximity to your customers. I think Nissan will give a lighter weight, smaller, better looking, and much higher performance version of the Z this time around since it is expected to pay tribute to the 240Z which raised the bar significantly once it was released.