NHRA rules...
5- point roll bar for:
13.49 or faster for convertibles other wise 11.49 or faster
at 9.99 or faster you get into full cages... Looks like you need a full cage in a convertible at 10.99...
this is from:
http://www.nhraonline.com/contacts/tech_faq.html
looks like IHRA rules are basically the same for the convertible now.... 13.49 but they require a 6 point roll bar..
some tracks may let you slide for like Test and Tune... .. but when it comes to safety, i follow the rules...