I say do it. You are already thinking about it, no reason to have any regrets, especially since it really isn't that much more cost.
(Full disclosure: I am moving your way and would like to see one in person before I order :D)
I have to say, every time I look at the 18's, I notice something new about it. It is really growing on me. Maybe a PP2 is in my future. Thanks for posting OP.
The Road and Track article has it.
http://www.roadandtrack.com/new-cars/future-cars/a13069178/2018-ford-mustang-gt-performance-package-level-2-info-photos/?src=nl&mag=rdt&list=nl_rdt_news&date=102317