Very pretty!
I have to ask though, what is it for? Just chainsaw competition? I can't imagine using that in the woods for long. Plus blowing the exhaust back at ya. How much more horsepower/rpms/gas mileage, etc are you theoretically expecting from using an expansion chamber like that?
A long time ago I guy I knew had a poulan where some or all of the exhaust (I forget now) got piped through the handle to keep your hands warm. Looking at your design..almost seems that something like that could be the handle itself...if you wanted a hot handle. Maybe put something else over the outside to insulate so it was just warm, and not redhot.