I'd suggest that you really want the FIA bars (the two vertical bars that support the windshield).
Take a look at these photos from a Boxster Spec roll-over. This was the result of minor contact between two cars at around 85mph. This car was the innocent driver - he was tapped in the rear, spun off track, caught in the dirt, and rolled. It doesn't happen often but it can happen and a cage is there for exactly that worst case scenario.
Most cage builders think that the cages they build without the FIA bars are more than adequate - but the pictures tell a different story...
