Roll cage perfromance

[(steven)]

I am looking at potentially fitting a roll cage to a track car. Originally I thought I would go for a half cage as the car needs to be road drivable (getting to the ring) and I don't want to have to wear a helmet everywhere I go.

The problem is for small cars (Read Pug 205, Saxo etc) how much benefit does a full cage bring. Specifically I worry about frontal impacts in small cars, as there isn't much room between yourself and the engine especially when strapped into a bucket seat.

Am I worrying about nothing or should I avoid small cars for track work unless I have a full cage.