i'm 6'3" with a 2001 Boxster S, which has factory option sport seats. Know what you mean about the head height and driving position.
I tend to have my seat lowest it will go, set slightly more forward than in my other car, and reclined slightly with the steering wheel fully extended. That way i have about an inch and a half between the seat headrest and the wind baffle.
As another poster pointed out, its a convertible car.
In a roll-over event in a non-race prepared car, there will be considerable impact damage that will likely negate ones health. The other thing that is more important is that on public roads its not the roll over but what you hit. Telegrpah poles, street lights, walls and worst off trees. Track events are usually limited in the possibilities of hitting obstructions.
http://www.dailymail.co.uk/news/worldnews/article-1309179/Driver-sliced-half-horrific-100mph-crash-drag-race-dead-mans-curve.html
Also as another poster mentioned, you'll really have to go some to roll a boxster.