I agree with DMcQuinn exhaust should not be a problem and the TT46 should be comparable to the OS46.

If you have removed the baffle in the muffler put it back in, it makes a big difference in the midrange and idle performance, the engine being reliable is more important than a few extra RPM at full throttle.
Since the engine is semi inverted try to make sure the fuel tank is not mounted to high in the plane.
Most will tell you the centerline of the tank should match the spray bar in the carb, I have found that the tank needs to be lower than that in relation to the spraybar or the engine will flood until the fuel level in the tank drops below the spraybar. This is particularly a problem at idle. Flooding at low throttle settings causes a lot of problems, the low end needle is set to lean to try to compensate for the flooding and this can result in virtually no range on the high speed needle since the high end is restricted by the low end.
The exhaust pressure to the tank varies with throttle setting so when the tank is full you set the needle valves so the engine runs semi correctly, as the fuel level drops below the spray bar level the engine will be to lean at less than full throttle because the exhaust pressure is low and gravity no longer siphons the fuel to the spraybar.
I don't know if what I have described fits your problem or not, but if your engine does not have at least one full turn of adjustability on the high speed needle where you can you adjust it from blubbering rich (and it will run for a prolonged period of time rich at full throttle) to peaked I would suspect that the low speed needle is to lean.
If fuel drips out of the carburator with a full tank of fuel when the plane is held horizontal then I think the tank is to high.
Good luck to you.