The hardtop itself and hardware is very easy to fit and install. 2 person job. Various postings here about the install process. The NER PCA allows Boxsters with out a hardtop at DE events; but you may need the BK roll bar extension. I'm sure owners from some other regions will respond as well.
For what its worth, hardtops are required in the Boxster spec racing class; it is not listed under the 'safety requirements', but under the body section.
http://www.boxsterspecracing.org/bsrrules.html
"Body:
Hard tops are mandatory and may be either factory or after market fiberglass replicas of the factory roof. No carbon fiber. Rear Window must be stock in appearance with no venting (can be lexan material)."
My feelings are that it would provide minimal added protection in a roll over with out a supplemental roll cage.
As for the added overall safety... I don't want to steer you in the wrong direction I will leave safety guidance to others with more knowledge of the hardtops construction and accidents.