Your exhaust run should not touch the hull at ANY point, but it has nothing to do with the risk of fire.
Noise and lots of it. Think of your hull as a large kettle drum that you happen to live ont he inside of. The thrud-thrud-thrud sound of the exhaust will reverberate beautifully throughout the vessel''s interior.
Even through foam lagging.
I recommend using the rubber exhaust hangers that are used on cars and trucs and building some support frames for them into several parts of the hull. This will suspend the exhaust an inch (or even 1/2 an inch will do) away form the hull at all points between the engine and the exhaust through-hull.
OUr boat had someone''s idea of a "clever" exhaust system when bought it too (not nearly as clever as the fuel system though, but that is another horror story). The sound of our 20hp Yanmar has dropped by about 60% since elevating the exhaust system away from the hull surfaces. It will get even quieter when I build the soundproofed companionway engine cover.... (I''ve been busy, okay?)