Similar concept to "inducers" run in the axial style pumps. The 1st scroll of the impeller helps to load the pump faster because it shortens the distance from the intake to the impeller. The first part of the twin screw/inducer also starts to rotate water for trailing edge of the impeller to make the pump more efficient. They also help the boat stay loaded in turbulent conditions, a lot of river race jet boats run them. Theoretically you won't pick up any top end speed unless you change the pitch of the impeller which most of the twin scrolls seem to do.
If you change the pitch of the impeller there is an exponential relationship in terms of power required to spin it. A small change and the stock engines can probably still reach the same RPM or maybe slightly more to pick up top end, too aggressive with the pitch and you won't have enough power to spin it fast enough and lose top end.
If you notice on the twin screw impellers the 1st part of the scroll is smaller just like the inducer setup.
Inducer Pre-Impeller Cavitation Reducer
www.berkeleyjet.com
I am not entirely positive why they aren't installed from the factory but I would guess cost?
There is a lot research into jet pumps and sizing of intake, suction, runner lengths, impeller placement, etc to create the ideal pressure zones way above my understanding.