Most HPDEs don't allow convertibles without full roll bars any more. Don't blame the organizers, blame their insurance. If they could have you participate as-is of course they would, they want as many cars as they can get.
(I guess Porsche does stand behind the safety of their cars!)
What is that supposed to mean? BMWCCA != BMW
Your best hope is that they'll make an exception for the 3 series since it's a hard top vert.