It's going to depend how good your cage is...if its well built, it's stronger than your roof anyways so cut the pillar out and you will be fine.

Circle Track Magazine built an asphalt enduro car awhile back and they cut the b pillar out with no problem.
My last two races at Grandview last year I had the driver side B pillar removed as well...although my car had already rolled so the roof wasn't doing anything.