If the hose could normally be done in 1.5 without additional fittings, adding a reducing coupler just adds a leak point. Why make it smaller than you have to?
Especially when you will eventually throw the jabsco head overboard
The mascerator head is fitted with a 1” discharge as standard (on the model I have)
And, on the tank, there is a 1” barb fitting.
So, it’s all straight through.
I don’t design heads… but Jabsco has some experience with this. They must have been comfortable with 1” port on a mascerator outlet. To be clear, the outlet on their stand alone mascerators is indeed 1”, with a 1.5” intake.
Lastly, the head install on the 49 is very tight where the hose exits. They actually fit an optional 90 degree swivel to get the hose route manageable