You may want to consider extending the Intumescent coating many feet behind the firewall on the belly.  A fire occurred on downwind in a Harmon Rocket flown by a well known aerobatic pilot. The plane was successfully landed but a hole had been burned in the belly.  The pilot, Vern Dahlman, suffered burns but died of complications from smoke inhalation.  This was probably less than a minute.

Steve Colwell