Just a couple of more things--
Some guys futz with the throttle curve to patch this setup a bit . again- smaller props are a good fix-
My own fix is easier - I run two large, open, non tuned cans on my 42% Radiocraft
The engine/ can/ prop combo ZDZ160J/JMB cans/14x30 ZM prop, is extremely quiet and having given the model to other fliers to try, all agree this setup is absolutely seamless in throttle response.-
The big difference in my setup and your in cowl muffler setup -is noise reduction. (mufflers and higher pitched two blades)
If you decide to try in fuselage cans --avoid any which require specific header lengths - these also tend to have a step in them if the prop load is not matched .