On a street lo-po sled your chances of vehicle fire are pretty slim...
However on a high vibration, exposed
oil line race car like the Cobra I see no reason to skimp on anything saftey related. Another thing to consider is at high speeds it takes that much longer to get the car stopped and get everyone out, grab the extinguisher, get the hood open...
BTW I don't consider the engine bay system enough, you should still have a hand extinguisher. (if anything to help to help the other guy get his car put out)