the bottom brace is still needed no matter what bumper is used..
i have split a 3mm chassis, mainly because of a hard crash but it was made worse because of how thick the chassis is, the brace sits to high where it mounts to the chassis, so the chassis is daylighting on the bottom. When the brace gets some impact it impacts the chassis midpoint causing it to split. I guess filing away the brace to allow the chassis to sit flush with the brace might work.. (this is with the 3mm chassis.