See below for the official NHRA requirements... to need a full cage, that would have to be one hell of a CRX.... A roll bar is required at 11.49... and if you're running good enough to need that, props......

When do I need a roll bar or roll cage, and what should it look like?
A roll bar is required in any convertible running 13.49 seconds or quicker in the quarter mile, and in other cars beginning at 11.49. The roll bar is accepted in vehicles running as quick as 10.00 second e.t., provided the stock firewall and floorboard is intact, other than for installation of wheel tubs.

If the floor and/or firewall has been modified, then a full roll cage is required beginning at a 10.99 e.t. A full roll cage is required in any vehicle running 9.99 seconds or quicker, and any vehicle running 135 mph or faster (regardless of e.t.)

The roll cage of any vehicle running 9.99 or quicker, or 135 mph or faster, must also be certified by NHRA every 3 years, and have a serialized sticker affixed prior to participation.

Here's the link:

http://www.nhraonline.com/contacts/tech_faq.html