A couple of things
There are a few things you can do, that are typically done to try to increase the heat in the flow to the cabin.
First, you can restrict the amount of flow -- you probably have a 2" sceet tube going from the intake to the heat muff and then to the cabin. At the intake, try restricting the amount of flow -- I have a disc with a 1" hole over the intake, so that cuts the flow to 1/4 what it would be with a full 2" tube.
Second, to improve the heat conduction off the exhaust pipe and into the air, fill the heat muff with some stainless steel wool, or what we in the USA call a "chore-boy" -- it is a small bundle of stainless steel braided wire used to clean pots and pans in the kitchen.
Let us know how it works out.