The Boxster enjoys unequal enforcement among PCA regions. I went through some of this recently where I live.
1) VIR (Virginia International Raceway)
requires the use of a BK bar, regardless of passing a broomstick test with the stock rollhoops
2) Potomac PCA allows you to run with a hardtop, even if you don't pass a broomstick test.
** Potomac PCA also judges your safety equipment by considering your equivalant PCA Club Race class. So. If you had a 2.5 boxster, but stuffed in a 996 motor, which would put you in a GT class for Club Race, it would be expected you have full cage and fire suppression. Yikes eh!
3) NNJR PCA only says you must pass the broomstick, hard top or not. If you need a BK bar to dos, then you must do so.
I my 914 days, I tried to run with the roof off at Watkins Glen. I was told I could do so, but that I must run with Arm Restraints. So. The roof went back on.
Things may be different where you plan to run, but consider looking at the rules for any/all groups you plan to run with, especially since the Boxster is an open car.