Hi, just thought I'd throw in my two cents here, a lot of which would be reinforcing some previous suggestions and comments.
First, don't write the car off just yet. As someone mentioned above, the seat height is adjustable, and may give you that inch back, or more, to at least provide you some psychological security. Even still don't write the car off, as there are other options to consider that may satisfy you.
As someone else mentioned, it's an open top car, and one can't trust some metal tubes, plastic, and cloth to protect you anywhere near the equivalent of a solid roof...I certainly don't. That being said, I'm sure you're aware that there is a removable hardtop for both generations of the Boxster. In my opinion, consider this unless you've absolutely got to have the wind in your hair at the touch of a button. Some would say 'why buy a convertible if you're not going to use it properly?', but ignoring that and assuming that the ragtop is not the only selling point for you, I'd say consider purchasing one. That would put you in the middle territory between a ragtop Boxster and a Cayman in terms of protection...and to me it looks so much better.
But I digress...
Also mentioned previously is the windshield frame and windshield, which is required to be able to withstand a rollover without completely buckling in order for a convertible to be roadworthy. Obviously that's not going to apply to every possible scenario other than whatever NHTSA considers a vanilla rollover, but there's some more peace of mind.
I'm between 5'7" and 5'8", not exactly in your territory, but I don't feel that my height affords me any more safety than your height. As someone else mentioned, should you ever find yourself in a rollover situation and you have time to do so, grab the wheel and hunker down. That's about the best you can do.
Lastly, I'd say, unless it's way out of your budget, look into an early Cayman. You'll get all the pros of a Boxster, and have a solid roof, no ifs, ands, or buts.
Hopefully we've all given you something to help you in your decision. Good luck, and if you do end up with one of these cars, be sure to come let us know.
